Nuclear suppressors of the [poky] cytoplasmic mutant in Neurospora crassa. III. Effects on other cytoplasmic mutants and on mitochondrial ribosome assembly in [poky].

We have previously isolated six non-allelic, nuclear mutations (sui loci) that partially suppress the growth, respiratory and cytochrome abnormalities of the extranuclear [poky] mutant. A comparison of the mitochondrial ribosome profiles of suppressed and unsuppressed [poky] strains revealed that five of the six suppressors alleviate at least partially the deficiency of mitochondrial small ribosomal subunits that is associated with the [poky] genotype. Six independently isolated Group 1 extranuclear mutants, namely [exn-1], [exn-2], [exn-4-a1, [stp-b1], [SG-1] and [SG-3-A1, which have growth and cytochrome phenotypes similar to [poky] also were found to be deficient in small subunits of mitochondrial ribosomes. Using cytochrome aa3 and b production as a criterion for mitochondrial protein synthesis, it could be shown that the nuclear su I suppressors of [poky] also suppress the other six Group I extranuclear mutants. However, differences in the efficiencies of suppression by suI suppressors suggest that at least some of Group I extrachromosomal mutants are not simply re-isolates of [poky], but represent distinct extranuclear mutations.