MATURASE AND ENDONUCLEASE FUNCTIONS DEPEND ON SEPARATE CONSERVED DOMAINS OF THE BIFUNCTIONAL PROTEIN ENCODED BY THE GROUP-I INTRON AI4-ALPHA OF YEAST MITOCHONDRIAL-DNA

Intron 4 alpha, (aI4 alpha) of the yeast mitochondrial COXI gene is a mobile group I intron that contains a reading frame encoding both the homing endonuclease I-SceII and a latent maturase capable of splicing both aI4 alpha and the fourth intron of the cytochrome b (COB) gene (b I4). The aI4 alpha reading frame is a member of a large gene family re cognized by the presence of related dodecapeptide sequence motifs call ed P1 and P2. In this study, missense mutations of P1 and P2 were plac ed in mitochondrial DNA by biolistic transformation. The effects of th e mutations on intron mobility, endonuclease I-SceII activity and matu rase function were tested. The mutations of P1 strongly affected mobil ity and endonuclease I-SceII activity, but had little or no effect on maturase function; mutations of P2 affected splicing but not mobility or endonuclease I-SceII activity. Surprisingly, the conditional (tempe rature-sensitive) mutations at P1 and P2 block one or the other functi on of the protein but not both. This study indicates that the two func tions depend on separate domains of the intron-encoded protein.