Transcriptional Control in Bacteria

Because the first committed step of gene expression is the transcription of the gene, a large fraction of genetic control takes place at this step, especially through the initiation of transcription. Genetic control schemes may be classified in several ways. 

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The end products of gene expression are proteins, mainly enzymes, and it is essential that their levels be strictly controlled. There are many potential sites of control in both bacteria and eukaryotes. DNA or gene amplification (Chap. 16) in eukaryotes is one way of responding to the demand for more of the protein product if there arc more copies of the gene, then transcription can occur at a faster rate. More often, control is effected at the level of cither transcription or translation, with the former probably being more important for both bacteria and eukaryotes. Transcriptional control in bacteria is particularly effective because of the very short half-life (a few minutes) of mRNA in such cells the half-life is longer in eukaryotes. The prototype for transcriptional control is the lactose operon in E. coli. 

Transcriptional control in bacteria is particularly effective because of the very short half life (a few minutes) of mRNA in such cells it is longer in eukaryotes. The prototype for transcriptional control is the lactose operon in E. coli. 

The mechanisms to initiate transcription in eukaryotes and prokaryotes are similar. As a comparison to control of transcription in eukaryotes some key features in transcriptional control in bacteria will be given. Prokaryotic cells contain only one type of RNA polymerase, which is responsible for synthesis of all types of RNA mRNA, rRNA,... 

Raibaud, O. Schwartz, M. (1984). Positive control of transcription initiation in bacteria. Annual Review of Genetics, 18, 173-206. 

T. M. Henkin. 2000. Transcription termination control in bacteria Curr. Opin. Microbiol. 3 149-153. (PubMed)... 

Genetic control in bacteria is primarily achieved by DNA repressor and stimulator proteins, which block or activate the coordinated transcription of gene clusters by binding to specific operator sites on the DNA. Many of these proteins bind in the DNA major groove with an a-helix. 

Crosa, J. H. (1997). Signal transduction and transcriptional and post transcriptional control of iron-regulated genes in bacteria, Microbiol. Mol. Biol. Rev., 61, 319-336. 

A major goal in recombinant DNA technology is the production of useful foreign proteins by bacteria, yeast, or other cultured cells. Protein synthesis depends upon both transcription and translation of the cloned genes and may also involve secretion of proteins from the host cells. The first step, transcription, is controlled to a major extent by the structures of promoters and other control elements in the DNA (Chapter 28). Since eukaryotic promoters often function poorly in bacteria, it is customary to put the cloned gene under the control of a strong bacterial or viral X promoter. The latter include the X promoter PL (Fig. 28-8) and the lac (Fig. 28-2) and trp promoters of E. coli. These are all available in cloning vehicles. 

There are three primary domains of life, represented by the bacteria, archaea, and eukaryotes. Some of the clearest evidence for the independent evolution of these three groups of organisms is found in the transcriptional apparatus. While the basic chemistry is the same, the details of initiation and control of transcription in bacteria and in eukaryotes are very different.2643 The archaea share characteristics of both bacteria and eukaryotes. Archaeal RNA polymerases have a complexity similar to that of eukaryotes and also share a similar mechanism of initiation of transcription 265 266b... 

The adenylate cyclases (AC) are a family of enzymes, which catalyze the synthesis of cyclic AMP (cAMP), from ATP. Cyclic AMP, a ubiquitous molecule in mammalian cells, plays a key role in controlling a vast number of biological processes, functioning as a major second messenger. The ACs are present in bacteria, where c-AMP plays a key role in the regulation of transcription in fungi, parasites and mammalian cells. The mammalians ACs (at least nine enzymes) are structurally unrelated to the bacterial ones consisting of 12 transmembrane helices and two cytoplasmic catalytic domains. They differ from each other in their... 

Another way in which gene expression is regulated is by translational control, where the rate of protein synthesis is controlled at the point of transcription of mRNA into polypeptides (Appendix 5.6). Generally, the majority of the control mechanisms in bacteria is at the transcriptional level. Translational control is less well understood and appears to be a secondary mechanism in bacteria, but it is thought to be very important in eukaryotic organisms. 

Eukaryotes potentially have many more opportunities for control of gene expression than do bacteria. For example, the cell could take advantage of control at the level of the processing of primary transcripts. It is known that RNA is not transported across the nuclear membrane until all introns are excised. A more subtle form of control could involve alternative modes of splicing a particular transcript. There are now examples known where this occurs to yield different mRNA molecules. Perhaps one of the best-known examples of yet another level of control in eukaryotes is that of translational control of globin synthesis. 

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