All sibdgenomic mRNAs of equine arteritis virus contain am common leader sequence

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 4α of the coxl gene encoding subunit I of cytochrome c oxidase (al4α). 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 oxl 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/mk− heteroplasms arise spontaneously at very high frequencies they provide a system that can be used to obtain mutants unable to form or maintain heteroplasmic cells.