ABSTRACT
https://www.niso.org/standards/z39-96/ns/oasis-exchange/table"> Transcription The first stage of converting the primary genetic information from the stable DNA code into protein is to transcribe the DNA into messenger RNA (mRNA). The RNA polymerases that carry this out bind to a DNA-coded signal (promoter) upstream of the gene to be transcribed. Genes in Bacteria can be transcribed alone (monocistronically) or with others as part of a polycistronic operon. Promoters RNA polymerase is composed of four polypeptides (αββ′σ). The core polymerase (αββ′) has a high processivity but low DNA affinity. The presence of sigma factor (σ) makes the holo-polymerase and confers DNA sequence specificity to the promoter. Once the RNA polymerase has bound, the sigma factor dissociates. The core polymerase then synthesizes RNA complementary to the lower (template) DNA strand. In E. coli, the promoter is made up of two conserved regions, the ‘-10’ or Pribnowbox, and ‘-35’. The numbers -10 and -35 refer to the number of bases the sequence is from the base where transcription starts. The -10(TATAAT)/-35(TTGACA) consensus promoter is not the only sequence of promoter in the genome, other sequences allow the binding of alternative sigma factors, which can control specialized groups of genes. These regulons can thus be switched on and off according to external changes in the cell’s environment. Termination of transcription The signal for termination of transcription is provided by a structure on the mRNA itself. Termination can be signaled by a stem-loop structure, or by the action of the protein Rho. Regulation of transcription All genes are regulated in some way at some stage during the cell cycle. In Bacteria this mainly happens in two ways: by derepression (where a protein bound to a promoter stopping transcription is removed and the gene is switched on) and attenuation (where the presence or absence of a substrate necessary for the function of the gene product governs the transcription of the gene itself). Less common is activation, where the presence of protein is used to switch a gene on. The lac operon The lac operon is made up of a promoter, an operator (a site on the DNA where regulatory proteins bind) and the genes lacZ, lacY, and lacA. Another promoter controls the transcription of lad, coding for the regulator (Lac repressor protein). When Lad is bound to the lacZYA operon operator, transcription is blocked and the cell is unable to produce (β-galactosidase. Allolactose (a by-product of the action of β-galactosidase on lactose) is the primary inducer of the operon. In this derepressed state, the lac promoter is relatively weak and only achieves full strength if the protein CRP binds as well. The trp operon The tryptophan (trp) operon is made up of five genes (trpEDCBA), regulated by the binding of the trp repressor complexed to tryptophan. A series of stem-loop structures can form, which regulate transcription of the operon by attenuation. Related topics (E2) Electron transport, oxidative phosphorylation, and β-oxidation of fatty acids (F5) Messenger RNA and translation (F6) Signal transduction and environmental sensing 125
