Group I and group II ribozymes as RNPs: clues to the past and guides to the future

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 im-pairing 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