Promiscuous exoribonucleases of Escherichia coli.

RNases play a central role in the metabolism of RNA. Among the many reactions catalyzed by these enzymes are those that separate individual entities of multicomponent transcripts, those that generate the mature 5' and 3' termini of RNAs, and those that remove introns. RNases also degrade RNA molecules that are no longer needed by the cell or that are damaged in some way, and under some conditions they also digest RNAs to supply nucleotides for continued RNA synthesis. As knowledge of the details of RNA metabolism has expanded, there has been a growing realization that many RNases with distinct specificities might be needed to catalyze the diverse reactions undergone by RNA molecules. Our understanding of this problem has progressed most rapidly in studies with Escherchia coli, so that for this organism close to 20 RNases are already known (17), and others undoubtedly exist. Exoribonucleases are a class of RNases that digest RNA molecules from their ends. The exoribonucleases currently known generate mononucleotide residues which can be reutilized for RNA synthesis. Exoribonucleases have been implicated in a number of RNA metabolic events, including RNA maturation and RNA degradation. In the case of RNA maturation, exoribonucleases participate in the final processing steps, trimming terminal residues from certain RNA precursors. Exoribonucleases also play a role in degrading unwanted RNA molecules to the mononucleotide level. In E. coli, two exoribonucleases that act nonspecifically in vitro on all types of RNA molecules, PNPase and RNase II, have been known for many years (45, 66). The properties of these two enzymes, and those of nonspecific exoribonucleases identified from other organisms, incorrectly colored the early thinking about this class of enzymes. However, in contrast to the small number of nonspecific, degradative exoribonucleases initially identified, it is now clear that a single cell can contain a large number of distinct proteins that function to digest RNA exonucleolytically. This minireview will discuss the information that has come to light in recent years concerning this family of enzymes from E. coli.