Messenger RNA stability and its role in control of gene expression in bacteria and phages.

The stability of mRNA in prokaryotes depends on multiple factors and it has not yet been possible to describe the process of mRNA degradation in terms of a unique pathway. However, important advances have been made in the past 10 years with the characterization of the cis-acting RNA elements and the trans-acting cellular proteins that control mRNA decay. The trans-acting proteins are mainly four nucleases, two endo- (RNase E and RNase III) and two exonucleases (PNPase and RNase II), and poly(A) polymerase. RNase E and PNPase are found in a multienzyme complex called the degradosome. In addition to the host nucleases, phage T4 encodes a specific endonuclease called RegB. The cis-acting elements that protect mRNA from degradation are stable stem-loops at the 5' end of the transcript and terminators or REP sequences at their 3' end. The rate-limiting step in mRNA decay is usually an initial endonucleolytic cleavage that often occurs at the 5' extremity. This initial step is followed by directional 3' to 5' degradation by the two exonucleases. Several examples, reviewed here, indicate that mRNA degradation is an important step at which gene expression can be controlled. This regulation can be either global, as in the case of growth rate-dependent control, or specific, in response to changes in the environmental conditions.