MITOCHONDRIAL RIBOSOME ASSEMBLY IN NEUROSPORA-CRASSA - MUTANTS WITH DEFECTS IN MITOCHONDRIAL RIBOSOME ASSEMBLY

Mitochondrial (mt) ribosome assembly and function was studied in 5 nuclear and 6 extranuclear mutants of N. crassa which had previously been characterized as deficient in cytochromes b and aa3. All 6 extranuclear mutants showed phenotypes similar to that previously described for the extranuclear [poky] mutant: small subunit-deficient with 19 S rRNA rapidly degraded. The nuclear mutants have the following phenotypes: 297-24 is mt small subunit-deficient with 19 S RNA rapidly degraded, 289-56 is mt small subunit-deficient but contains normal ratios of 19 S to 25 S RNA in whole mitochondria, 289-67 and 299-9 show defects in the processing of 25 S RNA leading to accumulation of a large precursor RNA, and 289-4 is deficient in large subunits although a substantial, but less than normal, amount of 25 S RNA is present in the mitochondria. New insight into the phenotypes of mt small subunit-deficient mutants is provided. Previous studies using chloramphenicol suggest that some defects in the assembly of mt small subunits may arise secondarily as a result of inhibition of mt protein synthesis. Three mutants (289-56, 289-67 and 299-9) appear to show such defects. These strains contain incomplete mt small subunits which sediment more slowly than normal and are deficient in at least 2 proteins, S-5 and S-9. Correlation of mutant phenotypes with rates of mt protein synthesis in the different strains suggests that mt protein synthesis must be decreased to < 1/2 of the wild-type rate before secondary defects in mt small subunit assembly are observed. This threshold value is much lower than that which leads to gross deficiencies of cytochromes b and aa3. Although several mutants have phenotypes suggestive of alterations in mt ribosomal proteins, no such alterations could be identified by 2-dimensional gel electrophoresis.