RNA-processing Nucleases

Abstract

In all living organisms there are biosynthetic pathways by which gene transcripts are converted step-by-step into their final functional form in cells. The several classes of functional RNAs, like mRNA, tRNA, or rRNA, each undergo processing events during their intracellular metabolism (Perry 1982). The evidence is overwhelming that for each type of RNA there are ribonucleases that systematically alter the size of the original gene transcript. In addition to size changes, other modifications also occur in RNA biosynthetic pathways (e.g., methylation, polyadenylation, CCA addition, and the acquisition of cap structures at 5′ termini). This article is concerned solely with the nature and mode of action of enzymes that alter the size of primary RNA transcripts and their processing intermediates.

Since every gene transcript contains RNA, the main distinguishing feature among transcripts is nucleotide sequence. It is probably sequence, therefore, that determines which biosynthetic pathway an RNA molecule enters—that for rRNA, mRNA, tRNA, or some other RNA species. It must also be sequence that ultimately determines which set of processing RNases acts on a particular gene transcript, although it may not be sequence alone that directly defines a cleavage site.

RNA-PROCESSING EVENTS
The involvement of RNases in RNA-processing events can be categorized as follows: (1) specific endonucleolytic cleavage of gene transcripts to alter their size; (2) specific exonucleolytic cleavage to perform small size reductions at either end of a molecule; or (3) specific rejoining of RNA molecules (ligation) associated with splicing of transcripts. The first two nucleolytic mechanisms are known...