Intragenic suppressors that restore the splicing and homing activities of the protein encoded by the second intron of the Saccharomyces capensis cyt b gene

The second (bi2) intron of the mitochondrial cyt b gene from Saccharomyces capensis encodes a bifunctional protein which acts both as a maturase, prom oting intron splicing, and as a homing-endonuclease, I-ScaI, promoting intr on mobility. In this work we isolated and characterized revertants from a r espiratory-deficient mutant in which both functions of the protein have bee n lost. Intragenic revertants resulted mainly from monosubstitutions in the mutated codon and in one case from a distant second site mutation. All nov el variants of the S. capensis bi2 intron-encoded protein are competent for the maturase activity but only two of them can partially complement the ho ming function.