Prokaryotic Regulation

Transcription

1 Prokaryotic Regulation Control of transcription initiation can be: Positive control increases transcription when activators bind DNA Negative control reduces transcription when repressors bind to DNA regulatory regions called operators

2 Prokaryotic Regulation Prokaryotic cells often respond to their environment by changes in gene expression. Genes involved in the same metabolic pathway are organized in operons. Some operons are induced when the metabolic pathway is needed. Some operons are repressed when the metabolic pathway is no longer needed.

3 Operon Structure: An operon is a single transcriptional unit that includes a series of structural genes, a promoter, and an operator.

4 Prokaryotic Regulation The lac operon contains genes for the use of lactose as an energy source. Regulatory regions of the operon include the CAP binding site, promoter, and the operator. The coding region contains genes for 3 enzymes: b-galactosidase, permease, and transacetylase

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6 Eukaryotic Regulation Many features of gene regulation are common to both bacterial and eukaryotic cells. For example, in both types of cells, DNA-binding proteins influence the ability of RNA polymerase to initiate transcription. However, there are also some differences: Eukaryotic genes are not organized into operons and are rarely transcribed together into a single mrna molecule; instead, each structural gene typically has its own promoter and is transcribed separately. Chromatin structure affects gene expression in eukaryotic cells ; DNA must unwind from the histone proteins before transcription can take place. Although both repressors and activators function in eukaryotic and bacterial gene regulation, activators seem to be more common in eukaryotic cells. The regulation of gene expression in eukaryotic cells is characterized by a greater diversity of mechanisms that act at different points in the transfer of information from DNA to protein.

7 - Chromatin Structure and Gene Regulation (modification of gene structure) Acetylation: The acetylation of histone proteins alters chromatin structure and permits some transcription factors to bind to DNA. Addition of acetyl groups (CH3CO)

8 Acetyl groups are added to histone proteins by acteyltransferase enzymes. Other enzymes called deacetylases strip acetyl groups from histones and restore chromatin repression. Certain transcription factors and other proteins that regulate transcription either have acteyltransferase activity or attract acteyltransferases to the DNA. Some transcription factors and other regulatory proteins are known to alter chromatin structure without acetylating histone proteins. These chromatinremodeling complexes bind directly to particular sites on DNA and reposition the nucleosomes, allowing transcription factors to bind to promoters and initiate transcription.

9 Methylation: In Eukaryotic DNA, cytosine bases are often methylated to form 5- methylcytosine. Heavily methylated DNA is associated with the repression of transcription There is an association between DNA methylation and the deacetylation of histones, both of which repress transcription. Methylation appears to attract deacetylases, which remove acetyl groups from the histone tails, stabilizing the nucleosome structure and repressing transcription.

10 -Transcriptional Control Transcription is an important level of control in eukaryotic cells, and this control requires a number of different types of proteins and regulatory elements. General transcription factors and RNA polymerase assemble into a basal transcription apparatus, which binds to a core promoter located immediately upstream of a gene. The basal transcription apparatus is capable of minimal levels of transcription. Transcriptional activator proteins (activators) are required to bring about normal levels of transcription. These proteins bind to a regulatory promoter, which is located upstream of the core promoter, and to enhancers, which may be located some distance from the gene.

11 -Transcriptional Control (continue) Coactivators and mediators which bind to transcription factors and bind to other parts of the transcription apparatus are also required for the function of transcription factors. Repressors which inhibit transcription, may bind to sequences in the regulatory promoter or to distant sequences called silencers (like enhancers). Repressors may compete with activators for DNA binding sites or may bind to sites near an activator site and prevent the activator from contacting the basal transcription apparatus or may have direct interference with the assembly of the basal transcription apparatus, thereby blocking the initiation of transcription.

12 Transcriptional activator proteins bind to sites on DNA and stimulate transcription. Most act by stimulating or stabilizing the assembly of the basal transcription apparatus.

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14 - Posttranscriptional Regulation - (Gene Control Through Messenger RNA Processing) Alternative splicing allows a pre-mrna to be spliced in multiple ways, generating different proteins in different tissues or at different times in development. Alternative splicing leads to the production of the small t antigen and the large T antigen in the mammalian virus SV40.

15 - Gene Control Through RNA Stability The amount of available mrna depends on both the rate of mrna synthesis and the rate of mrna degradation. Cellular RNA is degraded by ribonucleases, enzymes that specifically break down RNA. Mature mrna molecules have various half-lives depending on the gene and the location (tissue) of expression. The amount of polypeptide produced from a particular gene can be influenced by the half-life of the mrna molecules.

16 - RNA Silencing The expression of some genes may be suppressed through RNA silencing, also known as RNA interference and posttranscriptional gene silencing. Conclusion: sirnas produced from double-stranded RNA molecules affect gene expression.

17 - Translational and Posttranslational Control Ribosomes, aminoacyl trnas, initiation factors, and elongation factors are all required for the translation of mrna molecules. The availability of these components affects the rate of translation and therefore influences gene expression. Many eukaryotic proteins are extensively modified after translation by the selective cleavage and trimming of amino acids from the ends, by acetylation, or by the addition of phosphates, carboxyl groups, methyl groups, and carbohydrates to the protein). These modifications affect the transport, function, and activity of the proteins and have the capacity to affect gene expression.