SUMMARY

• 36.1 Cellular RNA Is Synthesized by RNA Polymerases

  All cellular RNA molecules are synthesized by RNA polymerases according to instructions given by DNA templates. The activated monomer substrates are ribonucleoside triphosphates. The direction of RNA synthesis is 5′ → 3′, as in DNA synthesis. RNA polymerases, unlike DNA polymerases, do not need a primer. RNA polymerase in E. coli is a multisubunit enzyme. The subunit composition of the ∼500-kDa holoenzyme is α₂ββ′σω and that of the core enzyme is α₂ββ′ω.

• 36.2 RNA Synthesis Comprises Three Stages

  Transcription is initiated at promoter sites consisting of two sequences, one centered near −10 and the other near −35; that is, 10 and 35 nucleotides away from the start site in the 5′ (upstream) direction, respectively. The consensus sequence of the −10 region is TATAAT. The σ subunit enables the holoenzyme to recognize promoter sites. RNA polymerase must unwind the template double helix for transcription to take place. Unwinding exposes some 17 bases on the template strand and sets the stage for the formation of the first phosphodiester linkage. RNA chains usually start with pppG or pppA. The σ subunit dissociates from the holoenzyme after the initiation of the new chain. Elongation takes place at transcription bubbles that move along the DNA template at a rate of about 50 nucleotides per second. The nascent RNA chain contains stop signals that end transcription. One stop signal is an RNA hairpin, which is followed by several U residues. A different stop signal is read by the rho protein, an ATPase.

  Although mRNA undergoes little posttranscriptional modification in bacteria, rRNA and tRNA are heavily processed. In E. coli, precursors of transfer RNA and ribosomal RNA are cleaved and chemically modified after transcription.

• 36.3 The lac Operon Illustrates the Control of Bacterial Gene Expression

  The lac operon is a well-studied example of gene regulation in E. coli. An operon is a coordinated unit of gene expression, consisting of regulatory elements and protein-encoding genes. In regard to the lac operon, three enzymes—β-galactosidase, a permease, and a transacetylase—facilitate the use of lactose as a fuel. The regulatory components include a regulatory gene that encodes the lac repressor protein and a DNA sequence called the operator site. In the absence of lactose, the repressor binds to the operator site, preventing the transcription of the protein-encoding genes. When lactose is present, a metabolite of lactose, allolactose, binds the repressor, causing it to leave the operator and permitting transcription.

  The lac operon is stimulated when the catabolite activator protein, also known as the cAMP response protein, binds near the lac operon promoter and stimulates the initiation of transcription. The binding takes place only when glucose is scarce.

  Another example of the regulation of gene expression observed in some bacterial species is quorum sensing. This process includes the release of chemicals called autoinducers into the medium surrounding the cells. Autoinducers are taken up by surrounding cells and activate the expression of genes, including those that promote the synthesis of more autoinducer. These chemically mediated social interactions allow these bacteria to change their gene-expression patterns in response to the number of other cells in their environments. Biofilms are complex communities of bacteria that are promoted by quorum-sensing mechanisms.