Processing of yeast mitochondrial RNA: Involvement of intramolecular hybrids in splicing of cob intron 4 RNA by mutation and reversion

Abstract

Revertants have been obtained from six mutants of the box9 cluster, which are supposed to be defective in RNA splicing as a result of alterations in a splice signal sequence. This sequence is in the 5′ part of intron 4 of the cob gene, 330 to 340 bp downstream from the 5′ splice site. Sequencing reveals that reversion to splicing competence is achieved by restoration of the wild-type box9 sequence; by creation of novel box9 sequences; and by introduction of a second site or suppressor mutation ( sup −) compensating for the effect of the primary box9- mutation. The sup − mutation alters a sequence in intron 4, 293 bp upstream from the box9 − primary mutation. The box9 sequence and this upstream sequence can base pair to form an intramolecular hybrid in intron RNA in which box9 − and sup − are compensatory base pair exchanges (G → A and C → U, respectively). Thus intramolecular hybrid structures of intron RNA are essential for RNA splicing.