12 RNA Maturation Nucleases

Abstract

I. INTRODUCTION
Most, if not all, RNAs are synthesized as precursor molecules that must undergo a series of maturation reactions to generate the functional form of the RNA. These include, first of all, reactions in which cotranscribed RNAs are separated from one another (e.g., the various species of ribosomal RNA, multimeric transfer RNAs, and messenger RNA-tRNA dual function transcripts). Second, the mature 5′ and 3′ termini of the RNA molecules are formed by cleavage (endonucleolytic) or trimming (exonucleolytic) reactions that remove precursor-specific sequences. In some cases, for RNA transcripts that contain introns, internal RNA regions also are removed by complex splicing reactions. Finally, some RNAs are not completely matured until base or sugar modifications or nucleotide additions (e.g., poly[A], –CCA) are incorporated into the molecule. All of these types of reactions, carried out on an ever-increasing number of diverse RNA molecules, have made RNA maturation an exceedingly complex, albeit exciting, area of research. Most of the reactions encompassing RNA maturation require the action of a ribonuclease. As the complexity and diversity of RNA maturation have unfolded, there has been a corresponding increase in our awareness of the many RNases necessary to carry out these reactions. This chapter focuses on RNA maturation pathways and on those RNases for which clear evidence, genetic or enzymological, exists for their involvement in RNA maturation. The many other RNases for which a definite role in maturation has not yet been established are not considered here or are mentioned only briefly.

II. THE STUDY OF RNA...