Control in prokaryotes

One mechanism of transcriptional control in prokaryotes, especially of several operons controlling the biosynthesis of amino acids, is attenuation. 

Be able to discuss the levels at which gene expression is controlled in prokaryotes. 

The G factor is a subunit of prokaryotic RNA polymerase. It directs the polymerase to specific promoters and is one of the ways that gene expression is controlled in prokaryotes. 

Nes, W.R. and McKean, M.L. The Biochemistry of Steroids and Other Isopentenoids Univorsity Park Press, Baltimore, MD, 1977 pp 37-57. Stanier, R.Y., Organisation and Control in Prokaryotic and Eukaryotic Cells, 1, Twentieth Symp. Soc. Gen. Microbiol., Cambridge Univ. Press, 1970. 

Activator proteins (and a few repressors) are important in eukaryotes, as they are in prokaryotes. The DNA sequences to which activator proteins bind in eiikaryotic DNA are called response elements. A few response elements are located within the promoter region (upstream promoter elements [UPE]), but most are outside the promoter and often clustered to form an enhancer region that allows control of gene expression by multiple signals (Figure 1-5-4). 

The nomenclature for transcription factors is confusing. Depending on their mode of action, various terms are in use both for the proteins themselves and for the DNA sequences to which they bind. If a factor blocks transcription, it is referred to as a repressor otherwise, it is called an inducer. DNA sequences to which regulatory proteins bind are referred to as control elements. In prokaryotes, control elements that serve as binding sites for RNA polymerases are called promoters, whereas repressor-binding sequences are usually called operators. Control elements that bind activating factors are termed enhancers, while elements that bind inhibiting factors are known as silencers. 

Controlled expression of genes coding for biologically active proteins in prokaryotes and eukaryotes, including transformed mammahan cells... 

In prokaryotes and eukaryotes, the expression of individual genes is controlled by activation or inhibition of RNA polymerase on each gene by transcription factors. 

The molecular and genetic relationship between enzyme induction and repression was clarified by the genetic research of Jacob and Monod at the Pasteur Institute, Paris (see reference l7>). Their classic work led them to develop the operon hypothesis for the control of protein synthesis in prokaryotes, which has since been verified by direct biochemical experiments. 

The problem of regulation in prokaryotes such as E. coli is far simpler than in complex eukaryotes because of the smaller number of genes. It seems likely that a system of this complexity (about 3,000 genes) should contain no more than 100-300 regulatory proteins because genes with related functions are often under the control of the same regulatory proteins. 

In eukaryotes, the tRNA genes exist as multiple copies and are transcribed by RNA polymerase III (RNA Pol III). As in prokaryotes, several tRNAs may be transcribed together to yield a single pre-tRNA molecule that is then processed to release the mature tRNAs. The promoters of eukaryotic tRNA genes are unusual in that the transcriptional control elements are located downstream (i.e. on the 3 side) of the transcriptional start site (at position +1). In fact they lie within the gene itself. Two such elements have been identified, called the A box and B box (Fig. 3). Transcription of the tRNA genes by RNA Pol III requires transcription factor IIIC (TFIIIC) as well as TFIIIB. THIIC binds to the A and B boxes whilst TFIIIB binds upstream of the A box. TFIIIB contains three subunits, one of which is TBP (TATA binding protein), the polypeptide required by all three eukaryotic RNA polymerases. 

Zinc-fingers are common in DNA-binding proteins of eukaryotes but are not found in prokaryotes. Examples of zinc-finger proteins include the RNA polymerase III transcription factor TFIIIA, steroid receptors, and some gene products that control development. The zinc-finger consists of pairs of cysteine and/or histidine residues within an a-helix. These residues bind tightly to a Zn2+ ion, which allows the a-helical amino acids to interact with specific sequences. See Figure 12-18. 

---