Mitochondrial class II introns encode proteins related to the reverse transcriptases of retroviruses

Abstract

Organelle introns share several distinctive features that set them apart from their counterparts in nuclear-encoded pre-messenger RNAs (reviewed in ref. 1): (1) their termini do not obey the GU… AG rule; (2) the introns are ‘structured’ (members of the same family or ‘class’ can theoretically adopt very similar RNA secondary conformations^1–3 and several of the postulated pairings have been confirmed by studies of splicing mutants and their revertants (see, for example, ref. 4); (3) many introns from both classes contain long open reading frames^5–10. We report here that the proteins potentially encoded by four class II introns are related to several RNA-dependent polymerases of viral and transposable element origins.