28 Translational Control in Prokaryotes

Bacteria and phages have extremely sophisticated ways to adapt the level of gene expression to growth condition changes or to specific needs at a given stage of their life cycle. A general model for gene regulation was proposed by Jacob and Monod (1961) that adequately explained many aspects of phage and bacterial regulation at the level of transcription but paid almost no attention to translation as a level at which gene expression could be regulated. In prokaryotes, protein-mediated translational regulation was discovered first in RNA bacteriophages (Lodish et al. 1964) and was later found to occur in DNA phages and in Escherichia coli . The first trans -acting RNA regulators were found in plasmids (Light and Molin 1981) and later in bacteria (Mizuno et al. 1984) and phages (Wu et al. 1987). The most significant recent finding is that translation (and transcription) can be regulated without the help of a regulatory RNA or protein, abandoning this cornerstone of the Jacob and Monod model. It is now accepted that mRNAs can act as direct sensors of the physical and/or metabolic state of the cell and modulate transcription or translation via switches in mRNA conformation. There are three different ways to temporally regulate gene expression at the translational level: through trans -acting proteins, through trans- acting RNAs, and through cis -acting mRNA elements acting as sensors. The three mechanisms are reviewed with a limited number of examples that illustrate the amazing diversity of these controls. Regulation of translation at the elongation/termination steps and at the level of...