Transcript Elongation and Termination in Bacteria

Abstract

Ribonucleic acid (RNA) molecules are elongated by the addition of nucleotides to the 3′ end of a nascent RNA by action of RNA polymerase . The substrates are the four ribonucleoside triphosphates. The order of their addition is directed by pairing with an unwound part of the template deoxyribonucleic acid (DNA) strand and is driven by the release of two of the phosphates as pyrophosphate. The rate of addition varies considerably from one nucleotide to the next, but the overall rate is ∼48 nt s −1 along genes encoding proteins, synchronised with the rate of translation. The elongation complex is very stable over most sequences. Termination of transcription can occur at an ‘intrinsic’ terminator – a set of sequences where the complex becomes unstable, or at a Rho‐dependent terminator, where the protein Rho acts on the RNA in the transcription complex to mediate its release. The regulation of expression of genes is controlled in many instances by the action of regulatory terminators (attenuators) or by the function of factors that affect the elongation rate and the action of terminators. Key Concepts: RNA polymerase unwinds ∼15 bp of DNA so that one strand can serve as a template for the assembly of an RNA transcript. The elongation complex of RNA polymerase, DNA and nascent RNA is very stable over most DNA sequences, allowing the synthesis of transcripts that are thousands of nucleotides long. Transcripts encoding protein are elongated with an average rate of 48 nt s −1 , which is synchronous with the rate of translation. The rate of step‐wise addition of nucleotides to the nascent RNA varies considerably from position to position on the DNA. Pauses in the elongation process serves as regulatory points. At the sequences of an intrinsic terminator the elongation complex becomes unstable allowing spontaneous release of a transcript. Termination of transcription is mediated in some cases by action of a factor called Rho, which functions by first binding to an exposed transcript followed by acting on the RNA in a reaction powered by ATP hydrolysis.