Critical base substitutions that affect the splicing and/or homing activities of the group I intron bi2 of yeast mitochondria

The second intron (bi2) of the cyt b gene from related Saccharomyces specie s has an extraordinarily conserved sequence and can have different function s in wild-type cells. The protein encoded by the S. cerevisiae intron funct ions as a maturase to promote intron splicing, while the homologous S. cape nsis intron encodes a bifunctional protein that acts both as a maturase and as a homing endonuclease (I-ScaI) promoting intron mobility. The protein e ncoded by intron bi2 belongs to a large gene family characterized by the pr esence of two conserved LAGLIDADG motifs (P1 and P2). In this study, we ana lysed a set of splicing-deficient mutants of the S. cerevisiae intron bi2 t hat carry non-directed mutations affecting the maturase activity, and a set of directed missense mutations introduced into the bifunctional protein en coded by the S. capensis intron. Analysis of these mutations has allowed id entification of the residues in the conserved P1 and P2 motifs which are cr ucial for splicing and homing activities. Moreover, several mutations which are located in the C-terminal part of the protein have been found to affec t both functions.