Role of lac Genes in Induction of 3-Galactosidase

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1 JOURNAL OF BACTEROLOGY, Jan. 1969, p Vol. 97, NO. 1 Copyright 1969 American Society for Microbiology Printed in U.S.A. Role of lac Genes in nduction of 3-Galactosidase Synthesis by Galactose' BARBARA LLANES2 AND ELZABETH McFALL Departmenit of Microbiology, New York Universitv School of Medicine, New York, New York 116 Received for publication 27 September 1968 Strain BL13, a laco mutant, synthesizes f3-galactosidase constitutively at a low rate. The enzyme is further inducible by D-galactose to the same differential rate as is seen in the presence of an optimal concentration of thiomethylgalactoside. lacy Mutants derived from strain BL13 are not inducible by galactose, although they synthesize f3-galactosidase at the low constitutive rate characteristic of the parent. Galactose is a weak inducer of f3-galactosidase synthesis in wild-type Escherichia coli K-12, but it is more effective when the wild type has been preinduced with isopropyl-f3-d-thiogalactoside. Nevertheless, the rise in the differential rate of synthesis in response to galactose in a preinduced wildtype culture is much lower than in strain BL13. Thus, two factors are involved in the induction of strain BL13 by galactose: the mutant operator and the constitutive permease. The operator has an altered sensitivity to the i productgalactose complex. The low constitutive level of permease enabled the cells, at the high concentrations of galactose used (5 X 1-2 M), to maintain a sufficient internal concentration for further induction. During an investigation into the cause of catabolite repression on 13-galactosidase synthesis, we isolated a spontaneous mutant of Escherichia coli K-12 which synthesizes,3-galactosidase constitutively at a low rate and is hyperinducible by galactose (12). Studies of strains harboring this mutation (denoted lacg) led to the conclusion that the repression of,b-galactosidase synthesis derived from galactose is distinct from the catabolite repression derived from glucose. n this paper, we present evidence that lacg is a mutation in the lac operator region. This mutation results in enhanced inducibility by galactose in the lacz+ lacy strains for two reasons. First, the altered operator is more readily released from the lac repressor by the action of galactose as an inducer than is the wild-type operator. Second, the operator mutation itself results in low constitutive synthesis of the lac permease, which transports sufficient galactose into lacg cells to induce further formation of the lac operon products. MATERALS AND METHODS The media, chemicals, preparation of and transduction by phage P1, and the jb-galactosidase assay are described in the accompanying paper (12). Bacterial strains. Table 1 shows the relevant genotypes of the numbered bacterial strains used in this study. Other strains used for particular experiments are described in the text. Strain 2PC, an F' strain used as the donor of Flac in dominance experiments, was provided by W. Maas (13). Strain W3828, a lac mutant with a short deletion in lacz (originally called lac D1D3; 4) was obtained from P. H. A. Sneath. J. Beckwith and J. Sadler kindly supplied strains xa-l and E13 and E322, respectively. The genetic nomenclature used in this paper adheres as closely as possible to the proposals of Demerec et al. (5). solation of lacy mutants. Three independent lac Y mutants were isolated from strain BL13 by mutagenic treatment followed by penicillin selection (9). For one mutant, ultraviolet (UV) mutagenesis was employed. A culture of strain BL13 in the logarithmic phase of growth in minimal medium was treated with UV light to a survival of 1-3. For the other two mutants, the nitrosoguanidine (MNNG) mutagenesis method of Adelberg, Mandel, and Chen (2; 2 ug of MNNG per ml for 3 min) was used. After mutagenesis, the cultures were grown in minimal medium to allow for the expression of the Lac phenotype. This was followed by cultivation in melibiose minimal medium with penicillin at 42 C. At 3 C, melibiose is transported into cells of E. coli K-12 by,3-galactoside permease (TMG permease ) and TMG permease 1 This work is being submitted by Barbara Lianes in partial fulfillment of the requirements for the Ph.D. degree. (16); the latter permease does not function at 42 C. 2Present address: Department of Chemistry, Princeton University, Princeton, N.J C, but do utilize this sugar at 3 C. Thus, lac Y mutants are unable to utilize melibiose at 223

2 224 LLANES AND MCFALL J. BAc-rEROL. Strain TABLE 1. Bacterial strainsa Relevant genotypeb W3828 lac dsda+ gal+ 2PC lac dsda+ thr leu thi/flac+ xa-1 lac BL12 lac+ dsda gal+ BL13 lacg dsda gal+ BL14 lacg dsda gal+/flac+ E13 lac lacz+ lacy+ met E322 lac lacz+ lacy met BL11 lacc dsda+ gal+c BL19 lacc2 dsda+ gal+ BL11 lacc3 dsda+ gal lac+ dsda+ gal+ a Relevant phenotypes are not given in this table, but are used in the text in accordance with the proposals of Demerec et al. (5). Specifically, LacG denotes low constitutive synthesis of the lac operon products and further inducibility by galactose. b Abbreviations for genetic loci: lac, lactose; dsda, sensitivity to D-serine (14); gal, galactose; thr, threonine; leu, leucine; thi, thiamine; met, methionine. c The designation lacc represents an unknown locus in the lac region responsible for constitutive production of lac operon products; i.e., either lac or laco. Preparation of stable merozygotes. Mating conditions, interruption of mating, and replica plating of patched colonies to determine the genetic constitution of recombinants approximated the techniques of Adelberg et al. (1). n addition to replica plating (11), recombinants were examined for sensitivity to the male-specific ribonucleic acid (RNA) bacteriophage MS2 (kindly supplied by C. Weissman). Acridine orange curing of Flac. To remove Flac from stable merozygotes, the acridine orange treatment of Hirota was employed (1). Acridine orange was used at a concentration of 25 to 3 gg/ml, with the initial cell density varying between 13 and 14 cells/ml. S-Galactoside permease assay.,3-galactoside permease activity was measured as the ability to accumulate '4C-thiomethylgalactoside (TMG). The method was that of Willson et al. (18), but L4C-labeled TMG was substituted for 35S-labeled f3-d-thiodigalactoside. Measurements of permease activity were performed on cells grown in minimal medium, either unsupplemented or supplemented with galactose or isopropylfl-d-thiogalactoside (PTG). The micromoles of TMG accumulated per gram of cells (dry weight) by a given culture grown in the presence of PTG was assigned an arbitrary value of 1. The specific activity of permease for the same culture grown under the same conditions but without PTG, in the presence or absence of galactose, was then expressed as a proportion of this arbitrary value. RESULTS Site of lacg mutation. The LacG phenotype might have resulted from a single mutation or from two separate mutations, one affecting induction of,-galactosidase by galactose and unrelated to a second resulting in constitutivity. Thus, although galactose is a very weak inducer for f3- galactosidase synthesis in wild-type E. coli, it is a very good inducer in certain gal mutants which can attain a high intracellular concentration of galactose (2). To determine whether the lacg mutation(s) resided in the lac region, strain W3828, which harbors an extensive deletion in lacz, was transduced to lac+ with P1 phage grown on strain BL1 (lacg, gal+). We examined 22 lacz+ transductants, all of which carried the two unselected markers, low constitutive synthesis of,b-galactosidase and hyperinducibility by galactose. Thus, either a single locus or two closely linked loci near the lacz region appear to be responsible for the LacG phenotype. Dominance studies on lacg. f the LacG phenotype were the result of a single mutation, this mutation might have occurred in lac, laco, or lacz. n lac, the result would be an altered lac repressor with a decreased affinity for the operator and an increased sensitivity to neutralization by galactose, as compared to the wild-type lac repressor. Such a mutation should be cis and trans recessive to lac+. A mutation in laco might result in a decreased affinity of the wild-type lac repressor for the altered operator. Since galactose can act as a weak inducer off,-galactosidase synthesis in wild-type strains of E. coli K-12, the decreased operator-repressor interaction in the laco mutant should facilitate the dissociation of the repressor from the operator by galactose. Such a mutation should be cis dominant, trans recessive to laco+. The mutation might have occurred in lacz, with the result that the f3-galactosidase produced catalyzes the formation of the "natural" inducer (3) from galactose or a metabolic derivative of galactose more efficiently than does the wild-type enzyme. Such a mutation should be cis and trans dominant. These alternatives would be distinguished on the basis of appropriate dominance tests. Accordingly, merodiploids of the genotype lacg/flac+ and lac+/flac+ were constructed, with strains BL13 and BL12 as recipients; strain 2PC was the Flac donor. A purified culture of each merodiploid was then tested in the following way. The cells were grown at 37 C with shaking in minimal medium containing no supplement, galactose, or TMG and were assayed periodically for f3-galactosidase activity. The re-

3 VOL. 97, 1969 NDUCTON OF 3-GALACTOSDASE 225 sults for the lacg/flac+ merodiploid designated BL14 are presented in Fig. 1, with the haploid BL13 for comparison. The differential rate of 3-galactosidase synthesis in the merodiploid with TMG as inducer was about twice that of the rate of the haploid BL13, whereas the two strains showed identical rates of synthesis in response to galactose. The high rate of synthesis in the presence of TMG in the merodiploid was expected, since more than one copy of the lacz+ gene is present per cell. Both cultures showed an initial lag corresponding to about one generation before induction by galactose occurred. The rate of constitutive synthesis was the same in both strains. As expected, the lacg+/flac+ merodiploid (not shown) was not induced by galactose. Since lacg is dominant to its wild-type allele, it is presumably not located in lad. t appears that the mutation is cis dominant only, although the absence of a trans gene dosage effect might be explained in other terms. A trans test, described later, confirmed that the dominance is cis only. To show that the BL14 culture used was a true merodiploid containing the lacg region on the chromosome and its wild-type allele on the episome, the culture was streaked out onto LB 15-5/ / OA A GROWTH(mqJml.) FG. 1. nduction of,-galactosidase synthesis in strailn BL4 (lacg/flac+, closed symbols) and in strain BL13 (lacg, open symbols). Each strain was pregrown in minimal medium to a cell density of 18 cells/ml and then was divided into three equal portions which received no supplement (, ), S X 12 Jf galactose (A, A), or S X 1J ml TMG (O, D). All cultures were incubated at 37 C with aeration and sampled periodically for,3-galactosidase assay and for cell density measuiement. agar at the end of the above experiment. After outgrowth of the cells, 1 isolated colonies were cultured in LB broth and examined for F' and chromosomal Lac phenotype. One sample of each culture was cured of Flac by acridine orange treatment (1). The resultant females were lacg, low constitutive for the production of f-galactosidase and further inducible by galactose or TMG. Another sample of each culture was also used as Flac donor with strain xa-1 (F- lacz) as recipient. The resultant xa-1/flac+ merodiploids were tested for the Lac phenotype. They were neither constitutive nor highly inducible by galactose, but were inducible by TMG. Hence, strain BL14 was indeed merodiploid for lacg and its wild-type allele. The lacg+/fac+ merodiploid was shown to be diploid for the lac region by analogous methods. From the results of the dominance experiment and of the induction kinetics, it appears that the low constitutive 13-galactosidase synthesis results from an alteration in the lac operator. Genetic basis for galactose induction in strains harboring lacg. f the LacG phenotype is due to a laco mutation, the induction by galactose could reasonably be explained as arising by either of two mechanisms or by a combination of both. First, the lac permease (TMG permease ) is known to transport galactose into cells of E. coli K-12 (6). t might be that the level of permease in lacg cells is high enough that, as in galk mutants, the cells can attain the threshold concentration of galactose necessary for induction of,-galactosidase. Second, there is considerable evidence to indicate that in laco mutants the affinity of the operator for the lac repressor is lessened (8; S. Cohn-Bourgeois, Ph.D. Thesis, Univ. of Paris, France, 1966). Galactose has a weak affinity for the lac repressor (7) and in wild-type strains has a low but measurable inductive effect (15). Thus, it might be expected that galactose would be an effective inducer in laco mutants. The roles of these two alternatives were evaluated by means of the following experiments. f the lac permease were the sole factor involved in galactose induction, galactose should induce,b-galactosidase synthesis in a preinduced lacg+ culture as effectively as it does in strains harboring lacg. Accordingly, two cultures of the lac+ strain BL12 (isogenic with strain BL13, lacg) were preinduced in minimal medium supplemented with PTG to specific activities about four times and two times higher than the level found in BL 13. The cells were then washed free from PTG and were resuspended in fresh minimal medium. They were allowed to grow for one doubling, then galactose was added as inducer. A culture of

4 226 LLANES AND MCFALL J. BACTEiuoL. BL13 was cultivated to a similar cell density, about 18/ml, but in the absence of PTG, and was also washed and resuspended in fresh minimal medium with galactose. Cultivation was continued, and samples were taken periodically for measurement of enzyme activity. The results of a typical experiment are presented in Fig. 2. n the preinduced wild-type strain, there was a lag in j3-galactosidase synthesis after the removal of PTG and the addition of galactose, then a sharp increase followed by a decline to a lower differential rate. n general, the higher the specific activity of j#-galactosidase at the beginning of an experiment, the shorter the initial lag and the greater the differential rate of synthesis during the burst. Preinduction sensitized the culture to some extent to further inducibility by galactose. However, the highest rate which was obtained after preinduction of the wild-type culture during numerous similar experiments (the slope of the line represented by solid triangles in Fig. 2) was still considerably lower than the rate induced by galactose in a lacg strain. t is apparent, therefore, that the presence of increased activities of the lac operon products is not, in itself, sufficient for maximal induction of,3-galactosidase synthesis by galactose. On the other hand, if the lac permease were not necessary to galactose induction, lacy derivatives of lacg strains should be as readily induced by galactose as the lac Y+ parent. Accordingly, three independently isolated lacy derivatives of strain BL13 were tested for inducibility by galactose (Fig. 3). None of the lac Y mutants were inducible 2COc % a (D 5-, * * GROWTH (mq.lrr.) FG. 2. nduction of f3-galactosidase synthesis in a preinduced lac+ culture. Two cultures ofstrain BL12 were grown overnight in minimal medium with PTG at concentrations of 7 X 1-6 M and 1J M, respectively. Each culture was harvested by centrifugation, diluted into the same medium in the absence ofinducer to cell density 5 X 17 cells/ml, allowed to grow to a cell density of about 18 cells/ml, and divided into two equal parts. Each portion received either no supplement (open figures) or 5 X 1-2 M galactose (closed figures). The initial,-galactosidase activity [units per milligram of cells (dry weight)] was 187 (, ) and 459 (A, A), respettively. ncubation was then continued and samples were taken periodically for t-galactosidase assay and cell density. For comparison, induction by galactose in strain BL13 is also shown. (X). 1. A B C -J U,3 2 4 a 4 A GROWTH (MG/ML) FG. 3. nduction of,8-galactosidase synthesis in three independent lacg lacy strains (A, B, C). Each strain was pregrown in minimal medium to a cell density of 18 cells/ml and then divided into three porlions which received no supplement (-), 5 x 1-2 M galactose (), or 5 X 1-4 M PTG (O). All cultures were further incubated at 37 C with aeration, and samples were removed for,3-galactosidase assay and cell density measurement.

5 VOL. 97, 1969 NDUCTON OF j3-galactosdase TABLE 2. j3-galactosidase and j3-galactoside permease synthesis in lacg lacz+lacy/flac+ merodiploidsa p-galactoside permeaseb 6-Galactosidasec Culture Synthesis with Synthesis with No Supplement Galactose PTG No supplement Galactose lacg lacz+ lacyl/flac lacg lacz+ lacy2/flac lacg lacz+ lac Y3/Flac lacg lacz+ lacy ,63 lac Three merodiploids of the constitution lacg lacz+ lac Y1/Flac+ were tested for j3-galactosidase and,8-galactoside permease activity after 3 hr of growth in either unsupplemented minimal medium or supplemented with 5 X 1-2 M galactose or 5 X 14 M PTG. b Basal or galactose-induced fl-galactoside permease activity is expressed relative to activity induced by PTG. c 3-Galactosidase is expressed as units per milligram of cells (dry weight). 227 by galactose, although all were readily induced by PTG. Thus, it appeared that the presence of a certain level of lac-permease in the cells is a necessary but not sufficient condition for induction by galactose. To definitively rule out the possibility that the LacG phenotype results from a mutation in lacz, a trans dominance experiment was performed. Three merodiploids lacg lacy lacz+/flacg+ lac Y+ lacz+ were formed by a mating between each of the lacg lac Y lacz+ strains described above and strain 2PC; these merodiploids were tested for their genetic composition in the same manner as the merodiploid described in the preceding section. The phenotype of these merodiploids, as shown in Table 2, was that expected if lacg were a laco mutation; i.e., a cis dominant constitutive. Thus, low constitutive synthesis of f-galactosidase, but not lac permease, was observed, although permease was readily induced by PTG. Galactose was no more effective as an inducer of permease in these merodiploids than in a wild-type strain. Thus, introduction of lac Y+ in trans position to lacy lacg did not restore the LacG phenotype. We also isolated five lacz derivatives of BL13 by MNNG mutagenesis, and tested them for induction of the lac permease by galactose. n each lacz mutant, the permease was as readily induced by galactose as in the parent strain. Thus, (3-galactosidase itself did not seem to be involved in induction of the lac operon products by galactose. However, it seemed of interest to examine the effect of galactose on the merodiploid lacg lacz lac Y+/Flac+. f the lac permease can cause accumulation of galactose to the threshold level necessary for induction with a wild-type lac operon, this merodiploid should form,3-galactosidase in response to galactose. Accordingly, a lacz derivative of strain BL13 was mated with strain 2PC, with selection for lac+. A lacg lacz lacy+/flac+ merodiploid obtained from this mating was cultivated in minimal medium to cell density 18 cells/ml. The culture was then divided into three equal parts; each part received PTG, galactose, or no supplement, and cultivation was continued. Samples were taken periodically for measurement of,b-galactosidase. A control culture of the merodiploid lacg+ lacz lac Y+/FlacG lacz+ lac Y+ was treated similarly, also after growth in minimal medium to cell density 18/ml. This merodiploid was formed by a cross between W3828 and a lacg lacz+ lacy+ homozygote derived from the diploid lacg lacz+ lac Y+/Flac+ (Fig. 4). Both cultures were induced equally well in response to PTG. Both also synthesized (-galactosidase in response to galactose, but the lag before synthesis was longer and the maximal rate of synthesis lower in the lacg lacz lac Y+/Flac+ merodiploid. This result indicates again that a high level of lac permease in the cell can enhance the inductive effect of galactose, but the full effect of the lacg mutation is not seen unless the three markers, lacg, lacz+, and lac Y+ are cis to each other. nduction by galactose in other low constitutive mutants. f weakening of the i-o interaction and presence of an appreciable level of the lac permease were responsible for galactose induction, it seemed reasonable that other laco mutants, and also lac mutants, might be inducible by galactose. n fact, Beverly Williams tested strain 2 OC and found it to be readily induced by galactose (personal communication). t was of interest then to test the effect of galactose in a low constitutive, hyperinducible, lac mutant. Strain E13 and its lacy derivative E322 both harbor the lac mutation it', which determines a thermolabile lac repressor. Both form,3-galacto-

6 228 LLANES AND MCFALL J. BACTEROL. E~~~~~~ *8-6[- 5 / / A 4 3- *, 7- a) a GROWTH (mg/ml) FG. 4. nduction of f3-galactosidase synithesis by galactose in a lacg lacz lacy+/flac+ straini (closed symbols) as compared to a lacz/f lacg lacz+ lacy+ strain (open symbols). The conditions of the experiment are the same as given in the legend to Fig. 2. The supplements are represented as: none (, ), galactose (A, A), PTG (, O) sidase at about 3.5% of the fully induced rate when cultivated at 37 C in the absence of any inducer. Both strains were cultivated in minimal 4 -J A medium into the logarithmic phase of growth, to a density of 18 cells/ml at 37 C. Each culture was then divided into two equal parts; one of which received galactose, the other no supplements. Cultivation was continued and samples were taken periodically for measurement of enzyme activity (Fig. 5 A). Strain E13 was readily induced by galactose, whereas its lac Y derivative was not. Thus, it appears that mutations in lac may also enhance the inductive effect of galactose, but only if permease activity is formed. During this investigation, we isolated from strain three other low constitutive mutants, BL11, BL19, and BL11. The nature of the mutations (lac or laco) was not determined. These three mutants were also readily induced by galactose (Fig. 5B, C), whereas there was no significant induction of the parent. nduction of constitutive mutants by galactose appears, therefore, to be a fairly general phenomenon. DSCUSSON Galactose, which has a low affinity for the lac repressor (7), is a weak inducer of f3-galactosidase synthesis in wild-type E. coli K-12. n galactokinaseless mutants, however, it is a very effective inducer. Wu showed that induction in the mutant strains, which requires the presence of galactose permease, is due to a high concentration of galac- B C Z 3 w c] O2- - C.,t r.5 'O A GROWTH(MG/ML) FG. 5. nduction of 3-galactosidase synthesis by galactose in several partially constitutive strains. Each culture was pregrown in minimal medium (for strains E13 and E322, methionine was added) and was subcultured at 18 cells/ml in either unsupplemented minimal medium (open symbols) or minimal medium supplemented with S X 1-2 mv galactose (closed symbols). For the various strains, the following symbols are used: (A) E322 (itl z+y-), A, A, and E13 (itlz+y+),, ; (B) BL11, A, A, and BL19,, ; (C) (lac+),,, and BL1OO, A, A.

7 VOL. 97, 1969 NDUCTON OF 3-GALACTOSDASE 229 tose in the cells (19). Thus, under conditions in which high levels of galactose can be accumulated, galactose can effectively neutralize the lac repressor. lacy+ Strains harboring laco and some lac mutations, as we have shown above, present another situation in which galactose becomes an effective inducer. With the repressor-operator interaction weakened, while the repressor-galactose interaction remains constant, the intracellular concentration of galactose necessary for induction is lower than in the wild type. Thus, one would predict that all hyperinducible lacy+ operator mutants should be inducible by galactose. However, only those hyperinducible lacy+ lac mutants in which the affinity of the lac repressor for galactose has not been weakened should be so inducible. Several other instances of increased efficiency of inducers or altered specificity of inducers have been described. Willson et al. (18) showed that the lowest concentration of PTG necessary to achieve maximal induction in a laco mutant is 1-fold less than in the wild type. Bourgeois showed that the pattern of response to a group of inducers is greatly altered in certain lac (is) mutants as compared to the wild type (Ph.D. Thesis, Univ. of Paris, France, 1966). Moreover, Williams and Paigen (17) described a number of "paradoxical compounds" which inhibit enzyme synthesis in lac mutants, but do induce laco mutants. t seems possible that the effects of these compounds may also derive from interactions with the lac repressor. ACKNOWLEDGMENTS This investigation was supported by Public Health Service research grant GM from the National nstitute of General Medical Sciences. E.M. is a research career development awardee of the U.S. Public Health Service (GM 739). B.L. was a predoctoral trainee of the National nstitutes of Health graduate training program. We are grateful to F. Jacob, who suggested the pre-induction experiment. LTERATURE CTED 1. Adelberg, E. A., and S. N. Burns Genetic variation in the sex factor of Escherichia coil. J. Bacteriol. 79: Adelberg, E. A., M. Mandel, and G. L. L. Chen Optimal conditions for mutagenesis by N-methyl-N'-nitro- N-nitrosoguanidine in Escherichia coli K12. Biochem. Biophys. Res. Commun. 18: Burstein, C., M. Cohn, A. Kepes, and J. Monod Role du lactose et des ses produits metaboliques dans l'induction de l'operon lactose chez Escherichia coli. Biochim. Biophys. Acta 95: Cook, A., and J. Lederberg Recombination studies of lactose nonfermenting mutants of Escherichia coli K12. Genetics 47: Demerec, M., A. Adelberg, A. J. Clark, and P. E. Hartman A proposal for a uniform nomenclature in bacterial genetics. Genetics 54: Ganesan, A. K., and B. Rotman Transport systems for galactose and galactosides in Escherichia coli.. Genetic determination and regulation of the methylogalactoside permease. J. Mol. Biol. 16: Gilbert, W., and B. Muller-Hill solation of the lac repressor. Proc. Nat. Acad. Sci. U.S. 56: Gilbert, W., and B. Muller-Hill The lac operator is DNA. Proc. Natl. Acad. Sci. U.S. 58: Gorini, L., and H. Kaufman Selecting bacterial mutants by the penicillin method. Science 131: Hirota, Y., and T. lijima Acriflavine as an effective agent for eliminating F-factor in Escherichia coli K12. Nature 18: Lederberg, J., and E. M. Lederberg Replica plating and indirect selection of bacterial mutants. J. Bacteriol. 63: Llanes, B., and E. McFall Effect of galactose on,- galactosidase synthesis in Escherichia coli. J. Bacteriol. 97: Maas, R Exclusion of an Flac episome by an Hfr gene. Proc. Natl. Acad. Sci. U.S. 5: McFall, E Mapping of the D-serine deaminase region in Escherichia coli K12. Genetics 55: Monod, J., and M. Cohn La biosynthese induite des enzymes (adaptation enzymatique), p n F. F. Nord (ed.), Advances in enzymology, vol. 13. nterscience Publishers, nc., New York. 16. Pardee, A., and L. Prestidge A second permease for methyl-thio-js-d-galactoside in Escherichia coli. Biochim. Biophys. Acta 1: Williams, B., and K. Paigen A group of compounds exerting paradoxical activity in the regulation of the lac operon. Biochem. Biophys. Res. Commun. 24: Willson, C., D. Perrin, M. Cohn, F. Jacob, and J. Monod Non-inducible mutants of the regulator gene in the "lactose" system of Escherichia coll. J. Mol. Biol. 8: Wu, H. C. P Role of the galactose transport system in the establishment of endogenous induction of the galactose operon in Escherichia colt. J. Mol. Biol. 24: Wu, H. C. P., and H. M. Kalckar Endogenous induction of the galactose operon in Escherichia coli K12. Proc; Natl. Acad. Sci. U.S. 55:

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