Mutation of a CCG sequence in the 5′-untranslated region of the mitochondrially encoded cytochrome b mRNA in Saccharomyces cerevisiae results in destabilization of the message and respiratory deficiency of the mutant strain. This phenotype mimics that of a mutation in the nuclear CBP1 gene. Here it is shown that overexpression of the nuclear CBT1 gene, due to a transposon insertion in the 5′-untranslated region, rescues the respiratory defects resulting from mutating the CCG sequence to ACG. Overexpressing alleles of CBT1 are allelic to soc1, a previously isolated suppressor of cbp1 ts-induced temperature sensitivity of respiratory growth. Quantitative primer extension analysis indicated that cbt1 null strains have defects in 5′-end processing of precursor cytochrome b mRNA to the mature form. Cbt1p is also required for stabilizing the mature cytochrome b mRNA after 5′ processing.
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