EVOLUTIONARY RELATIONSHIPS AMONG GROUP-II INTRON-ENCODED PROTEINS AND IDENTIFICATION OF A CONSERVED DOMAIN THAT MAY BE RELATED TO MATURASE FUNCTION

Many group II introns encode reverse transcriptase-like proteins that potentially function in intron mobility and RNA splicing. We compared 34 intron-encoded open reading frames and four related open reading frames that are not encoded in introns. Many of these open reading frames have a reverse transcriptase-like domain, followed by an additional conserved domain X, and a Zn2+-finger-like region. Some open reading frames have lost conserved sequence blocks or key amino acids characteristic of functional reverse transcriptases, and some lack the Zn2+-finger-like region. The open reading frames encoded by the chloroplast tRNA(Lys) genes and the related Epifagus virginiana matK open reading frame lack a Zn2+-finger-like region and have only remnants of a reverse transcriptase-like domain, but retain a readily identifiable domain X. Several findings lead us to speculate that domain X may function in binding of the intron RNA during reverse transcription and RNA splicing. Overall, our findings are consistent with the hypothesis that all of the known group II intron open reading frames evolved from an ancestral open reading frame, which contained reverse transcriptase, X, and Zn2+-finger-like domains, and that the reverse transcriptase and Zn2+-finger-like domains were lost in some cases. The retention of domain X in most proteins may reflect an essential function in RNA splicing, which is independent of the reverse transcriptase activity of these proteins.