18 Group I and Group II Ribozymes as RNPs: Clues to the Past and Guides to the Future

Abstract

Group I and group II introns are not only catalytic RNAs, but also mobile genetic elements. The success of these introns as mobile elements almost certainly relates to their innate self-splicing capability, which enables them to propagate by inserting into host genes while only minimally impairing gene expression. Nevertheless, both types of introns have become dependent on proteins for efficient splicing in vivo to help fold the intron RNA into the catalytically active structure.

Although group I and group II introns have very different structures and splicing mechanisms (Chapter 13), there are striking parallels in the evolution of their protein-assisted splicing reactions. For example, the splicing factors for both types of introns include intron-encoded as well as cellular proteins, and the intron-encoded proteins, DNA endonucleases for group I introns and reverse transcriptases (RTs) for group II introns, also function in intron mobility. In addition, excised group I and group II intron RNAs remain associated with splicing factors in RNP particles, which can then cleave and insert into cellular RNA or DNA target sites by reverse splicing. The need to control this deleterious ribozyme activity may have been an evolutionary driving force favoring mutations that impaired self-splicing activity and resulted in dependence on protein factors (Nikolcheva and Woodson 1997).

In this chapter, we review protein-assisted reactions of group I and group II introns. These studies illustrate how proteins facilitate RNA folding and catalysis and provide unique insights into how splicing mechanisms evolve. A recurring theme, first developed in a previous review...