Abstract
The complexity of RNA metabolism has become much more apparent in recent years. First of all, it is now clear that there are many more types of RNA molecules present in cells than the original classes of rRNA, tRNA and mRNA. Secondly, most, if not all, of these RNA molecules are initially synthesized as precursors that must be processed to generate the mature, functional species. In addition, some of these functional RNAs undergo other turnover or modification reactions that further alter their structure. Finally, RNA molecules are ultimately degraded, and these degradative reactions proceed at different rates among classes of RNA molecules and even among members of the same class. These latter findings add an additional level of complexity to RNA metabolism because they imply that regulatory processes may be involved in the differential stability of RNA molecules.