Characterization of a Maternal T-Box Gene in Ciona intestinalis

Developmental Biology 225, 169 178 (2000) doi:10.1006/dbio.2000.9815, available online at http://www.idealibrary.com on Characterization of a Maternal T-Box Gene in Ciona intestinalis Albert Erives 1 and Michael Levine Division of Genetics & Development, Department of Molecular and Cell Biology, University of California at Berkeley, 401 Barker Hall, Berkeley, California 94720 A new T-box gene, CiVegTR, was isolated in the ascidian Ciona intestinalis. CiVegTR maternal RNAs become localized to the vegetal cytoplasm of fertilized eggs and are incorporated into muscle lineages derived from the B4.1 blastomere. The CiVegTR protein binds to specific sequences within a minimal, 262-bp enhancer that mediates Ci-snail expression in the tail muscles. Mutations in these binding sites abolish expression from an otherwise normal lacz reporter gene in electroporated embryos. In addition to the previously identified AC-core E-box sequences, T-box recognition sequences are conserved in the promoter regions of many genes expressed in B4.1 lineages in both Ciona and the distantly related ascidian Halocynthia. These results suggest that CiVegTR encodes a component of the classical muscle determinant that was first identified in ascidians nearly 100 years ago. 2000 Academic Press INTRODUCTION 1 Present address: 156-29, Division of Biology, California Institute of Technology, Pasadena, CA 91125. Studies on ascidians provided the first evidence for localized determinants in animal development (Chabry, 1887; Conklin, 1905). Chabry showed that the destruction of particular blastomeres led to the specific loss of muscle derivatives (Chabry, 1887). Subsequent lineage studies by Conklin established a tight correlation between the distribution of yellow crescent and muscle differentiation in Styela (Conklin, 1905). The yellow crescent becomes localized to the vegetal cytoplasm shortly after fertilization and is ultimately inherited by the two B4.1 blastomeres that form most of the tail muscles in the ascidian tadpole. More recent studies have employed a combination of histochemical staining and cellular inhibitors to provide definitive evidence for the existence of one or more muscle determinants in the ascidian egg (Jeffery and Meier, 1983; Meedel et al., 1987; Nishida, 1992). Current techniques provide new opportunities for the identification of this determinant(s). In particular, the advent of an efficient electroporation method for introducing transgenic DNA into developing Ciona embryos has permitted the detailed characterization of cis-regulatory sequences that control tissue-specific patterns of gene expression (Corbo et al., 1997). This method led to the identification of a minimal, 434-bp notochord-specific enhancer in the promoter region of the Ciona Brachyury gene. A Ciona ortholog of the Drosophila Snail repressor (Ci-sna) was found to repress this enhancer. Ci-sna is expressed in the developing tail muscles (Erives et al., 1998), where it is important for restricting Ci-Bra expression to the developing notochord (Fujiwara et al., 1998). Ci-sna is activated early during muscle specification (32-cell stage), at the time when maternal determinants first activate zygotic genes (Erives et al., 1998). We have launched a detailed analysis of Ci-sna regulation to identify these determinants. The present study identifies a 262-bp enhancer from the Ci-sna 5 flanking region that is sufficient to mediate expression in derivatives of the B4.1 blastomeres. This enhancer contains two conserved sequence motifs that are also present in the regulatory regions of muscle-specific genes in the distantly related ascidian Halocynthia. One of the motifs corresponds to a specialized E-box sequence (CAACTG), whereas the other contains conserved residues recognized by different T-box DNA binding proteins (GT- GNNA). Mutations in either motif diminish or abolish the expression driven by otherwise normal Ci-sna/lacZ transgenes. A candidate gene approach was used to identify regulatory factors that might interact with the T-box motifs within the Ci-sna enhancer. Previous studies identified VegT as a maternal T-box 0012-1606/00 $35.00 Copyright 2000 by Academic Press All rights of reproduction in any form reserved. 169

170 Erives and Levine FIG. 1. 262-bp muscle enhancer. (a) (Left) Tail-bud-stage Ciona embryo that was electroporated at the 1-cell stage with a lacz reporter gene containing a Ci-sna/Ci-fkh fusion promoter. The 262-bp Ci-sna enhancer extends from 618 and 356 bp, while the Ci-fkh promoter region extends to approx 180 bp upstream of the transcribed region (Erives et al., 1998). lacz expression is detected in the tail muscles and trunk mesenchyme. (Right) Same as the left except that the electroporated embryo was fixed at the 32- to 44-cell stage and hybridized

A Maternal T-Box Gene in Ciona intestinalis 171 transcription factor that is localized to the vegetal cytoplasm of Xenopus embryos (Zhang et al., 1998; Zhang and King, 1996). The T-domain of Xenopus VegT was used to isolate related genes in Ciona. One of these, CiVegTR (VegT-related), encodes maternal RNAs that are localized to the vegetal cytoplasm of fertilized eggs. A GST VegTR fusion protein is shown to bind the critical T-box motifs contained within the 262-bp muscle enhancer. Taken together, the in situ localization studies, DNA binding assays, and transgenic analyses suggest that CiVegTR encodes a component of the classical muscle determinant in ascidians. MATERIALS AND METHODS Ciona electroporations were done exactly as described previously (Corbo et al., 1997). RNA in situ hybridizations were done with digoxigenin-labeled antisense RNA probes exactly as described in Erives et al. (1998). The lacz hybridization probe used in Fig. 1 is the same as described in the previous Ci-sna study (Erives et al., 1998). Mutations were created within the T-box binding sites using the two-step PCR method with Pfu polymerase. Mutagenic oligonucleotides that contain either the single A 3 G mutation or the single G 3 A mutation within otherwise normal T-box sequences (as indicated in the text) were synthesized. These mutagenic primers also contain at least 20 nucleotides flanking the substitution. Mutagenized templates were confirmed by sequence analysis using the blackdog primer specific to the psp1.72-27 plasmid (Erives et al., 1998). CiVegTR was isolated by screening a Ciona genomic DNA library via low-stringency hybridization with the following probes. The DNA binding domain of the Xenopus VegT cdna Xombi was amplified using the X5-tbox oligo (5 -CTC TGG ACC CAG TTC CAC CAG G) and the X3-tbox oligo (5 -CCG GAA TCC TTT AGC AAA TGG G). The amplified product was purified on an agarose gel, and half of the eluate was used in a second reaction containing only the X5-tbox primer. Fifty nanograms of this product was labeled using [ - 32 P]dCTP and the Random Prime-It II kit (Stratagene). The probe was fractionated on a P-10 spin column and then incubated with genomic filters at 37 C for 36 h. CiVegTR in situ hybridization probes were made using a 1.2-kb 3 RACE fragment that includes the entire T-box domain (Figs. 4a, 4c, and 4e 4h) and a 311-bp PCR fragment containing the 3 end of the T-box domain (Figs. 4b and 4d). Eggs from three or four adults were harvested, fertilized, and washed. The eggs were split into two batches and placed in 10 ml of 0.20- m-filtered seawater for 12 min. One pool contained 1.2 M nocodazole, while the other pool contained untreated seawater. One-cell embryos were enzymatically dechorionated and passed through five dishes of seawater over a span of 23 min prior to fixation and hybridization. Internal deletions within the full-length promoter were obtained by cleaving a restriction site between E5, E6, and E7, digesting the ends with exonuclease III, and then blunt-end ligating the product. Exo 51 removes an 500-bp fragment encompassing the ClaI, NruI, BssHII, NdeI, and NheI sites while leaving the ScaI and EcoRI sites intact. RESULTS Previous studies identified a 737-bp DNA fragment from the Ci-sna 5 regulatory region that is sufficient to direct the restricted expression of a lacz reporter gene in the tail muscles of electroporated embryos (Erives et al., 1998). A 262-bp sequence contained within this 737-bp regulatory region is also sufficient to mediate lacz expression within derivatives of the B4.1 blastomeres, including the trunk mesenchyme and primary tail muscles (Fig. 1a, left). Coupled electroporations and in situ hybridization assays indicate that this 262-bp Ci-sna/lacZ reporter gene is activated at the 32- to 44-cell stage, at about the time when the endogenous gene is first expressed (Fig. 1a, right). Thus, the 262-bp enhancer appears to be sufficient to direct the authentic Ci-sna expression pattern. This conclusion is supported by the analysis of additional Ci-sna/lacZ fusion genes, as described below. The full-length Ci-sna promoter region containing 3.2 kb of 5 flanking sequence directs intense lacz expression in B4.1 lineages (the tail muscles), the CNS, and the progenitors of the tadpole muscles (Fig. 1b, left). This fusion gene is strongly activated in the B4.1 lineages at the 32- to 44-cell stage (Fig. 1b, right). However, a small internal deletion that removes the minimal, 262-bp enhancer results in a severe reduction in the activities of the full-length promoter (Fig. 1c; compare with 1b; see Materials and Methods). These results suggest that the activation of Ci-sna in the B4.1 lineages depends primarily on the 262-bp enhancer, although there may be additional, weak musclespecific elements located elsewhere in the promoter region. The 262-bp Ci-sna enhancer contains a variety of with a digoxigenin-labeled lacz antisense RNA probe. By this method, reporter gene expression can be detected at about the time when the endogenous Ci-sna gene is activated. (b) (Left) Late tail-bud-stage Ciona embryo that was electroporated with a lacz fusion gene 3.2 kb of the Ci-sna 5 flanking region. Intense reporter gene expression is detected in the tail muscles, trunk mesenchyme, adult muscle precursor cells, and CNS. (Right) Same as the left except that the lacz reporter gene was visualized via in situ hybridization at the 32- to 44-cell stage. Intense staining is detected in the progenitors of the tail muscles and mesenchyme. (c) (Left) Same as b (left) except that a small internal deletion was created within the full-length promoter (Exo 51), which removes the 262-bp enhancer. The internal deletion results in reduced staining in tail-bud embryos. (Right) Same as the left except that the lacz reporter gene was visualized via in situ hybridization. There is a severe loss of expression in 32- to 44-cell embryos compared with the wild-type Ci-sna promoter region (compare with b, right).

172 Erives and Levine FIG. 2. T-box binding elements in the Ci-sna enhancer. (a) Complete sequence of the 262-bp Ci-sna enhancer. Putative recognition sequences are indicated, including three E boxes (E5, E6, and E7) and two T-box motifs (T2 and T3). (b) lacz staining pattern obtained after electroporation of a fusion gene containing point mutations in the T2 and T3 T-box motifs within the 262-bp enhancer. The A residues at position 8 of the recognition sequences were mutagenized. Only residual staining is detected in the tail muscles (short arrow) and mesenchyme (long arrow). (c) Same as b except that mutations were created in the G residues of position 5 within the two T-box motifs. There is a complete loss of staining. (d) T-box sequences are conserved in the promoter regions of muscle-specific genes of another ascidian, Halocynthia (Hr). Asterisks indicate C residues of an AC-core E box, similar to the E5, E6, and E7 sequences contained within the 262-bp Ci-sna enhancer. Underlined sequences indicate mutations in the HrMA4a promoter region that abolish expression of a transgene (Satou and Satoh, 1996).

A Maternal T-Box Gene in Ciona intestinalis 173 potential recognition motifs, including three AC-core E-box sequences (E5, E6, and E7) and two T-box sequences (T2 and T3), which contain good matches (8/10 and 7/10, respectively) to the optimal Brachyury half-site, AGGTGTGAAA (Fig. 2a; see Kispert and Herman, 1993). Mutations in the conserved A residues at position 8 of the T2 and T3 sites cause a severe reduction in the expression mediated by the 262-bp/lacZ reporter gene (Fig. 2b; compare with Fig. 1a, left). This A residue is conserved in a number of potential T-box motifs present in the promoter regions of muscle-specific genes in both Ciona (e.g., Ci-sna) and Halocynthia (Hr; summarized in Fig. 2d). Mutations in the conserved G residue at position 5 cause a complete loss in the activity of the 262-bp enhancer (Fig. 2c). This G residue is located within the highly conserved GTG motif seen in all of the putative muscle-specific T boxes (Fig. 2d). The preceding results suggest that a T-box transcription factor is essential for Ci-sna expression in the B4.1 muscle lineages. This factor cannot correspond to the Ciona Brachyury protein, since it is expressed exclusively in notochord lineages (Corbo et al., 1997; Takahashi et al., 1999). A good candidate for the B4.1 determinant is an ascidian ortholog of the Xenopus VegT gene, which is maternally expressed and plays an important role in patterning the animal vegetal axis of the early embryo (Zhang et al., 1998; Zhang and King, 1996). However, a putative VegT/Tbx6 ortholog in Halocynthia (As-T2) is not maternally expressed (Mitani et al., 1999; Yasuo et al., 1996), whereas the only previously identified maternal T-box gene in ascidians, the Halocynthia As-mT gene, encodes transcripts that are uniformly distributed throughout the fertilized egg (Takada et al., 1998). In order to identify new T-box genes a Ciona genomic DNA library was screened with the T-box region of a Xenopus VegT cdna (see Materials and Methods). One of the genes that were identified, CiVegTR, is most closely related to mouse Tbx15 (Fig. 3a). The only other T-box gene that was identified in this screen corresponds to a Ciona ortholog of omb (data not shown). CiVegTR contains a short stretch of amino acid residues located C-terminal to the last -helix that are conserved with members of the VegT family (Fig. 3a, underlining). The entire T-box DNA binding domain from the CiVegTR gene was fused in-frame with GST and used in gel shift assays (see Materials and Methods). The fusion protein binds with high affinity to the T2 motif contained within the minimal 262-bp Ci-sna B4.1 enhancer (Fig. 3b, lane2). The addition of thrombin, which removes the GST motif, reduces the size of the shifted complex (lane 3). Substituting the A residue at position 8 causes a slight reduction in the binding of the GST CiVegTR fusion protein (lane 4). In contrast, altering the conserved G residue at position 5 causes a severe loss in binding (lane 5). The GST CiVegTR fusion protein also binds the T3 motif in the Ci-sna enhancer (Fig. 3b, lane 7), although with lower affinity than T2. Altering the A residue at position 8 drastically reduces binding to T3 (lane 9, compare with lane7), while disruption of the conserved G residue at position 5 virtually eliminates binding (lane 10, compare with T2 lane 5). Another putative T-box motif identified in the Ci-sna promoter region, T1, also binds the GST CiVegTR fusion protein (lane 11). The preceding analyses establish a close correlation between the DNA binding activities of CiVegTR in vitro (Fig. 3) and the expression of the 262-bp Ci-sna enhancer in vivo (Figs. 2b and 2c). For example, mutations in the A residues at position 8 reduce binding to T3 (but not T2) and cause a reduction of lacz expression, while mutations in the G residues at position 5 nearly eliminate CiVegTR binding to both T2 and T3 and abolish lacz expression. Further evidence that CiVegTR corresponds to a component of the muscle determinant was obtained by conducting in situ hybridization assays (Fig. 4). A digoxigeninlabeled CiVegTR single-stranded RNA probe was hybridized with eggs and embryos (Corbo et al., 1997). The antisense RNA probe consistently stained unfertilized eggs, suggesting that the gene is maternally expressed (Fig. 4a). Approximately 30 min after fertilization, the hybridization signal becomes localized to the vegetal cytoplasm. The most intense staining is detected in close proximity with the clear ooplasm in the vegetalmost regions that contains the myoplasm (Fig. 4b, left, horizontal arrow). The hybridization signal becomes more tightly localized within the vegetal cytoplasm approx 40 min after fertilization (Fig. 4c). Transcripts become separated into two vegetal domains during the first cleavage, approx 50 min after fertilization (Fig. 4d). This localization persists in the two blastomeres after cleavage is complete (Fig. 4e). There is a gradual diminishment in the hybridization signal during the 4- and 8-cell stages. The residual maternal CiVegTR RNAs that persist in 8-cell embryos exhibit a vegetal-to-animal distribution, with peak staining in the vegetalmost B4.1 blastomere (arrow, Fig. 4f) and slightly weaker expression in the A4.1 blastomere. Weak staining is sometimes detected in the b4.2 blastomere. There is no staining above background levels in 16-cell and 32-cell embryos (Fig. 4g; data not shown). These results suggest that CiVegTR is maternally expressed and not transcribed from the zygotic genome during early embryogenesis. The localization of maternal Vg1 transcripts to the vegetal cytoplasm of Xenopus eggs can be disrupted with the microtubule inhibitor nocodazole (Deshler et al., 1997). The incubation of Ciona eggs with nocodazole has been shown to block the cortical contractions required for the localization of the myoplasm to the vegetal pole (Roegiers et al., 1999). Nocodazole treatment (see Material and Methods) specifically blocks the localization of CiVegTR RNAs in fertilized eggs, so that only diffuse staining can be

174 Erives and Levine FIG. 3. CiVegTR sequence and DNA binding activities. (a) 284 aa region of the CiVegTR protein that includes the entire T domain. This is the sequence that was fused to GST and used in gel shifts (see below). The arrowheads indicate known intron positions, gray highlights conserved T-box family residues, red highlights residues that are specifically conserved between members of the VegT superfamily but not CiBra. The black bars beneath the sequences indicate -sheets while the white bars show -helices (Muller and Herrmann, 1997). The conserved alanine residue at position 195 is critical for interactions with the distal A nucleotide (position 8) in the recognition sequence.

A Maternal T-Box Gene in Ciona intestinalis 175 detected upon hybridization with the RNA probe (Fig. 4h, right). In contrast, untreated control eggs at the same stage (40 min) exhibit tight localization of the hybridization signal to the vegetal cytoplasm (Fig. 4h, left). These results suggest that microtubules are required for the localization of CiVegTR maternal RNAs within the vegetal cytoplasm. DISCUSSION We have presented evidence that CiVegTR might encode a component of the classical muscle determinant in ascidians. Three independent experiments are consistent with this conclusion. First, the minimal 262-bp B4.1 enhancer contains essential T-box recognition sites. Second, single nucleotide substitutions indicate a close correlation between CiVegTR binding in vitro and Ci-sna expression in vivo. Third, CiVegTR maternal transcripts exhibit the same type of localization pattern as the distribution of yellow crescent in Styela (Conklin, 1905). Potential T-box recognition elements were identified within the minimal B4.1 enhancer based on a search for optimal Brachyury binding sites. However, CiVegTR appears to interact with related, but somewhat distinct, recognition sequences (Fig. 2d). For example, the A residue at position 8 is critical for Brachyury but is not essential for the binding of CiVegTR to T2 (see lanes 4 and 9 of Fig. 3b). Additional experiments will be required to determine the basis of T-box specificity. However, A residues at positions 9 and 10 may be more important for the binding of CiVegTR than Brachyury. Moreover, T-box specificity is likely to involve enhancer context, as shown for Hox proteins (Li and McGinnis, 1999; Passner et al., 1999; Ryoo and Mann, 1999). Specific protein protein interactions within the Cisna B4.1 enhancer might favor the binding of CiVegTR over other T-box proteins. Both Ci-sna and a number of muscle-specific actin (HrMA) and myosin (HrMHC1) genes in Halocynthia (Satoh et al., 1996; Satou and Satoh, 1996) are activated in B4.1 derivatives at the 32-cell stage (summarized in Fig. 5). Ciona and Halocynthia belong to different orders of ascidians, Enterogona and Pleurogona, respectively, but nonetheless share two closely linked sequence motifs, the T box and a specialized E box (CAACTG) (Erives et al., 1998). Mutations in either motif (Fig. 2; Erives et al., 1998) eliminate expression from Ci-sna/lacZ transgenes. Thus, regulatory proteins that bind these motifs appear to be equally important in muscle specification. E boxes are often recognized by bhlh proteins (Murre et al., 1989), although most members of this family, including MyoD and Myogenin, preferentially bind E boxes with GC core motifs and not the AC motif seen in the ascidian promoters (Blackwell and Weintraub, 1990). It is possible that a divergent bhlh protein, such as Paraxis (Barnes et al., 1997; Burgess et al., 1995), Meso1 (Blanar et al., 1995; Buchberger et al., 1998), or Mesogenin (Yoon et al., 2000), might interact with CiVegTR to specify paraxial mesoderm in vertebrates and tail muscles in ascidians. There are also examples of zinc-finger transcription factors that bind E-box sequences. The Drosophila Snail repressor binds an AC-core E box-like motif (Ip et al., 1992), while the Ciona Snail repressor binds to the related sequence: CAAGTG (Fujiwara et al., 1998). It is therefore conceivable that an ascidian zinc finger transcription factor works in concert with CiVegTR to specify muscle lineages. In this regard we note that the zinc finger gene Manx is important for coordinating tail formation in the ascidian Mogula (Swalla and Jeffery, 1996). Crystallographic studies suggest that Ala195, an amino acid residue conserved in all T-box proteins, directly contacts the T nucleotide opposite A at position 8 (Muller and Herrmann, 1997). A loop emanating away from the main body of the T-box DNA binding domain contacts the phosphate backbone at positions 9 10 (Muller and Herrmann, 1997) and might interact with additional transcription factors within the 262-bp Ci-sna enhancer. In this regard we note that the T1 and T4 T-box motifs within the HrMA4a promoter region contain AC-core E boxes starting at position 10 (GCGTGATAA- CAACTG and ACGTGCGAACAACTG, respectively; Fig. 2d). A weak potential T-box motif in the Ci-sna enhancer, T4, is similarly spaced relative to the critical Ci-sna E7 site (CTGTGGCGACAACTG; see Fig. 5). Perhaps cooperative interactions between CiVegTR and E-box DNA binding proteins facilitate occupancy and activation of the Ci-sna cis-regulatory DNA in muscle lineages. Ala214 in the Brachyury subfamily of T-box proteins corresponds to a Gly in other members of the Tbx family and is important for interactions with the G residue at position 3 (5 -AGGTGNNA) in the recognition sequence (Muller and Herrmann, 1997). CiVegTR is most similar to MmTbx15 (176 aa/272 aa or 64%), ChTbx6L (145 aa/224 aa or 64%), DrTbx16 (138 aa/208 aa or 65%), and VegT (143 aa/226 aa or 63%) (19). (b) A GST CiVegTR fusion protein was mixed with double-stranded oligonucleotides that contain normal and mutant versions of T2 (lanes 1 5), T3 (lanes 6 10), or T1, which is a potential T-box binding site that maps outside the 262-bp Ci-sna enhancer. The fusion protein binds all three sequences (lanes 2, 7, 11). Limiting amounts of thrombin cause partial proteolysis of the fusion protein, resulting in a smaller protein DNA complex (lanes 3 and 8). Mutations in the conserved G residue at position 3 of the recognition sequence diminishes or eliminates binding (lanes 5 and 10). In contrast, mutations in the A residue at position 8 result in a reduced binding to T3 (lane 9), but have virtually no effect on the binding activity of T2 (lane 4).

176 Erives and Levine FIG. 4. CiVegTR expression patterns. A digoxigenin-labeled CiVegTR antisense RNA probe was hybridized to staged Ciona embryos. Stained embryos are oriented with the animal pole up and vegetal down. (a) Unfertilized oocyte. Diffuse staining is detected throughout the ooplasm. (b) 1-cell embryos approx 30 min after fertilization. Staining is detected in the vegetal cytoplasm just above the clear myoplasm in cortical regions (arrow, left embryo). Cortical contractions can be detected along the animal vegetal axis (vertical arrows, right embryo). (c) 1-cell embryo approx 40 min after fertilization. Staining becomes localized to a region of the vegetal cytoplasm that is inherited by the B4.1 blastomeres. (d) 1-cell embryo approx 50 min after fertilization. (e) 2-cell stage. (f) 8-cell stage, signal strongest in vegetal half. The differences in blastomere sizes, the regions of clear cytoplasm, and the various cleavage plane configurations that the cells make with one another are all established diagnostic features of cell identity, which make possible accurate localization of transcripts to specific blastomeres. (g) 16-cell stage, left is vegetal, right is animal, top is dorsal, and bottom is ventral. (h) Left, 40 min postfertilization staged control; right, 40 min postfertilization embryo treated with microtubule inhibitor nocodazole.

A Maternal T-Box Gene in Ciona intestinalis 177 FIG. 5. Conservation of E-box and T-box motifs in ascidian tail muscle genes. Putative T-box binding sites are conserved in the promoter regions of a number of muscle-specific genes in Halocynthia and Ciona. These are clustered sites which are closely linked to a specific E-box motif (AC core) located within the Ci-sna 262-bp enhancer. We propose that protein protein interactions between a T-box protein and AC-core E-box binding factors are facilitated by these linked sites and are essential for muscle specification in ascidians. CiVegTR is proposed to operate early at these sites although these sites may also be functional for a late-acting T box such as As-T2. ACKNOWLEDGMENTS We thank Christian Sardet for many helpful discussions and advice, including the suggestion to perform the nocodazole experiment. We thank Richard Harland for suggesting a closer look at VegT in Ciona muscle formation. A.E. is a predoctoral trainee of the NIH. This work was funded by a grant from the NSF (IBN- 9514138) and the France Berkeley Fund to M.L. and C. Sardet. REFERENCES Barnes, G. L., Alexander, P. G., Hsu, C. W., Mariani, B. D., and Tuan, R. S. (1997). Cloning and characterization of chicken Paraxis: A regulator of paraxial mesoderm development and somite formation. Dev. Biol. 189, 95 111. Blackwell, T. K., and Weintraub, H. (1990). Differences and similarities in DNA-binding preferences of MyoD and E2A protein complexes revealed by binding site selection. Science 250, 1104 1110. Blanar, M. A., Crossley, P. H., Peters, K. G., Steingrimsson, E., Copeland, N. G., Jenkins, N. A., Martin, G. R., and Rutter, W. J. (1995). Meso1, a basic-helix-loop-helix protein involved in mammalian presomitic mesoderm development. Proc. Natl. Acad. Sci. USA 92, 5870 5874. Buchberger, A., Seidl, K., Klein, C., Eberhardt, H., and Arnold, H. H. (1998). cmeso-1, a novel bhlh transcription factor, is involved in somite formation in chicken embryos. Dev. Biol. 199, 201 215. Burgess, R., Cserjesi, P., Ligon, K. L., and Olson, E. N. (1995). Paraxis: A basic helix-loop-helix protein expressed in paraxial mesoderm and developing somites. Dev. Biol. 168, 296 306. Chabry, L. (1887). Contribution a l embryologie normale teratologique des ascidies simples. J. Anat. Physiol. Norm. Pathol. 23, 167 231. Conklin, E. G. (1905). Organ-forming substances in the eggs of ascidians. Biol. Bull. 8, 205 230. Corbo, J. C., Levine, M., and Zeller, R. W. (1997). Characterization of a notochord-specific enhancer from the Brachyury promoter region of the ascidian, Ciona intestinalis. Development 124, 589 602. Deshler, J. O., Highett, M. I., and Schnapp, B. J. (1997). Localization of Xenopus Vg1 mrna by Vera protein and the endoplasmic reticulum. Science 276, 1128 1131. Erives, A., Corbo, J. C., and Levine, M. (1998). Lineage-specific regulation of the Ciona snail gene in the embryonic mesoderm and neuroectoderm. Dev. Biol. 194, 213 225. Fujiwara, S., Corbo, J. C., and Levine, M. (1998). The snail repressor establishes a muscle/notochord boundary in the Ciona embryo. Development 125, 2511 2520. Gray, S., Szymanski, P., and Levine, M. (1994). Short-range repression permits multiple enhancers to function autonomously within a complex promoter. Genes Dev. 8, 1829 1838. Ip, Y. T., Park, R. E., Kosman, D., Bier, E., and Levine, M. (1992). The dorsal gradient morphogen regulates stripes of rhomboid expression in the presumptive neuroectoderm of the Drosophila embryo. Genes Dev. 6, 1728 1739. Jeffery, W. R., and Meier, S. (1983). A yellow crescent cytoskeletal domain in ascidian eggs and its role in early development. Dev. Biol. 96, 125 143. Li, X., and McGinnis, W. (1999). Activity regulation of Hox proteins, a mechanism for altering functional specificity in development and evolution. Proc. Natl. Acad. Sci. USA 96, 6802 6807. Meedel, T. H., Crowther, R. J., and Whittaker, J. R. (1987). Determinative properties of muscle lineages in ascidian embryos. Development 100, 245 260. Mitani, Y., Takahashi, H., and Satoh, N. (1999). An ascidian T-box gene As-T2 is related to the Tbx6 subfamily and is associated

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