The putative GTPase encoded by MTG3 functions in a novel pathway for regulating assembly of the small subunit of yeast mitochondrial ribosomes.

Very little is known about biogenesis of mitochondrial ribosomes. The GTPases encoded by the nuclear MTG1 and MTG2 genes of Saccharomyces cerevisiae have been reported to play a role in assembly of the ribosomal 54 S subunit. In the present study biochemical screens of a collection of respiratory deficient yeast mutants have enabled us to identify a third gene essential for expression of mitochondrial ribosomes. This gene codes for a member of the YqeH family of GTPases, which we have named MTG3 in keeping with the earlier convention. Mutations in MTG3 cause the accumulation of the 15 S rRNA precursor, previously shown to have an 80-nucleotide 5' extension. Sucrose gradient sedimentation of mitochondrial ribosomes from temperature-sensitive mtg3 mutants grown at the permissive and restrictive temperatures, combined with immunobloting with subunit-specific antibodies, indicate that Mtg3p is required for assembly of the 30 S but not 54 S ribosomal subunit. The respiratory deficient growth phenotype of an mtg3 null mutant is partially rescued by overexpression of the Mrpl4p constituent located at the peptide exit site of the 54 S subunit. The rescue is accompanied by an increase in processed 15 S rRNA. This suggests that Mtg3p and Mrpl4p jointly regulate assembly of the small subunit by modulating processing of the 15 S rRNA precursor.