NOVEL HYBRID MATURASES IN UNSTABLE PSEUDOREVERTANTS OF MATURASELESS MUTANTS OF YEAST MITOCHONDRIAL-DNA

Unstable pseudorevertants of mitochondrial mutants of Saccharomyces cerevisiae lacking the maturase function encoded by the fourth intron of the cytochrome b gene (bl4) were isolated. They were found to be heteroplasmic cells owing their regained ability to respire (and grow on glycerol medium) to the presence of a rearranged (rho) mtDNA that contains an in-frame fusion of the reading frames of the group I introns bl4 and intron 4a of the coxl gene encoding subunit I of cytochrome c oxidase (al4<). The products of those gene fusions suppress the bl4 maturase deficiency still present in those heteroplasmic cells. Similar heteroplasmic pseudorevertants of a group II maturaseless mutant of the first Intron of the coxl gene were characterized; they result from partial deletion of the coxl gene that fuses the reading frames of introns 1 and 2. These heteroplasms provide independent support for the existence of RNA maturases encoded by group I and group II introns. Also, since the pettte/mltheteroplasms arise spontaneously at very high frequencies they provide a system that can be used to obtain mutants unable to form or maintain heteroplasmic cells. © 1990 Oxford University Press.