IN-VITRO MUTAGENESIS OF THE MITOCHONDRIAL LEUCYL TRANSFER-RNA SYNTHETASE OF SACCHAROMYCES-CEREVISIAE SHOWS THAT THE SUPPRESSOR ACTIVITY OF THE MUTANT PROTEINS IS RELATED TO THE SPLICING FUNCTION OF THE WILD-TYPE PROTEIN

The NAM2 gene of Saccharomyces cerevisiae encodes the mitochondrial le ucyl tRNA synthetase (mLRS), which is necessary for the excision of th e fourth intron of the mitochondrial cytb gene (bI4) and the fourth in tron of the mitochondrial coxI gene (aI4), as well as for mitochondria l protein synthesis. Some dominant mutant alleles of the gene are able to suppress mutations that inactivate the bI4 maturase, which is esse ntial for the excision of the introns aI4 and bI4. Here we report muta genesis studies which focus on the splicing and suppressor functions o f the protein. Small deletions in the C-terminal region of the protein preferentially reduce the splicing, but not the synthetase activty; a nd all the C-terminal deletions tested abolish the suppressor activity . Mutations which increase the volume of the residue at position 240 i n the wild-type mLRS without introducing a charge, lead to a suppresso r activity. The mutant 238C, which is located in the suppressor region , has a reduced synthetase activity and no detectable splicing activit y. These data show that the splicing and suppressor functions are link ed and that the suppressor activity of the mutant alleles results from a modification of the wild-type splicing activity.