Small Regulatory RNAs in Bacteria
In recent years, small regulatory RNAs have been discovered at a staggering rate both in prokaryotes and eukaryotes. By now it is clear that post transcriptional regulation of gene expression mediated by such RNAs is the rule rather than as previously believed the exception. In this chapter, we focus on small RNAs (sRNAs) encoded by bacterial chromosomes. The strategies for their discovery, their biological roles, and their mechanisms of action are discussed. Even though the number of well characterized sRNAs in, for example, the best studied model enterobacterium Escherichia coli, is still small, the emerging pattern suggests that antisense mechanisms predominate. In terms of their roles in bacterial physiology, most of these RNAs appear to be involved in stress response regulation. Some other examples indicate functions in regulation of virulence. Two aspects of sRNA mediated control arising from recent observations are addressed as well. Firstly, some sRNAs need proteins (notably Hfq) as helpers in their antisense activities at this point the reason for this requirement is not understood. Secondly, only limited sequence complementarity is generally observed in antisense target RNA pairs. This raises the fundamental question of how specific recognition is accomplished, and what the structure/sequence determinants for rapid and productive interaction are.