Mitochondrial gene expression in a nuclear mutant of Neurospora deficient in large subunits of mitochondrial ribosomes

Abstract

A new cytochrome a and b deficient nuclear mutant of Neurospora crassa, cyt-U-28, is defective in the assembly of large subunits of mitochondrial ribosomes. Nonetheless, this mutant overproduces apparently normal small subunits of mitochondrial ribosomes, even though it should be deficient for the S5 ribosomal protein required for assembly of the particles beyond the CAP30S stage. The mitochondria of cyt-U-28 indeed synthesize only small amounts of most mitochondrial polypeptides, including cytochrome oxidase subunits I, II, and III, contain very low amounts of the normal seven-polypeptide cytochrome oxidase complex, and, unlike the organelles from other cytochrome a deficient mutants, do not accumulate the nuclear-encoded cytochrome oxidase subunits 5 and 6. Nonetheless, the mutant markedly oversynthesizes a mitochondrial protein that comigrates with subunit 9 of the mitochondrial ATPase on S DS–polyacrylamide electrophoresis gels. The overproduction of this protein and the accumulation of mature small subunits of mitochondrial ribosomes indicate that the cyt-U-28 mutant preferentially expresses two mitochondrial genes, one coding for ATPase subunit 9, the other for the S5 ribosomal protein.Key words: Neurospora, mitochondria, ribosomes, protein synthesis.