Expression of the "split gene" cob in yeast mtDNA. Nuclear mutations specifically block the excision of different introns from its primary transcript.

Five nuclear mutants falling into five different complementation groups are shown to block the maturation of long form mitochondrial cob RNA at five different processing steps. At the same time they prevent complete processing of the oxi 3 RNA, thus exhibiting the same phenotype as mitochondrial box mutants (cyt b- and oxi 3-). The different nuclear factors in question have varying ranges of specificity for the removal of introns from cob RNA, from only one to at the most three introns. Two mutated nuclear elements are shown to be specific for the processing of introns present only in the long form cob gene. One such mutation shows, as expected, no deleterious effect on the processing of the short form cob RNA exchanged into the mutant via cytoduction. The role of nuclear coded factors in the possible translation or activity of introncoded products ("maturases") is discussed for two mutants. Striking parallels are found between diverse polypeptide products, presumably translated from accumulated cob RNA intermediates, in pet- and mit- mutants blocked in the excision of the same intron.