Group I and group II introns are remarkable not only as ribozymes that catalyze their own splicing, but also as mobile genetic elements. These so-called "infectious introns" are phylogenetic ally widespread, but of idiosyncratic distribution, reflecting the development of varied and efficient mechanisms for their dispersal. The "homing" of group I introns-i.e. transfer from intron-containing to intronless alleles of the same gene-has been well characterized in both eukaryotic and bacteriophage systems and proceeds via intron-encoded site-specific endonucleases. Interestingly, related endonucle? ases also exist in an archaeal intron, and as insertions in protein-coding sequences in all three kingdoms. In contrast, group II intron homing, which is less well characterized, may proceed by a different mechanism, using intron-encoded reverse-transcriptase-like proteins. In addition, both group I and group II introns appear capable of transposition to other locations, perhaps even across phylogenetic lines. Autonomous intron-related elements, which are suspected of being cDNA copies of excised introns, may playa role in such mobility and horizontal transfer events. Remarkably, in addition to mobility, both group I intron endonucleases and group II intron reverse-trans? criptase-like proteins also function in RNA splicing. Research on mobile group I and group II introns has revealed relationships between introns and other mobile elements, provided new insight into the evolution of introns and splicing mechanisms, and may be giving glimpses ofthe origins of retroviruses and other retroid elements.
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