MUTATION OF A HIGHLY CONSERVED BASE IN THE YEAST MITOCHONDRIAL 21S RIBOSOMAL-RNA RESTRICTS RIBOSOMAL FRAMESHIFTING

A mutation shown to cause resistance to chloramphenicol in Saccharomyc es cerevisiae was mapped to the central loop in domain V of the yeast mitochondrial 21S rRNA. The mutant 21S rRNA has a basepair exchange fr om U-2677 (corresponding to U-2504 in Escherichia coli) to C-2677, whi ch significantly reduces rightward frameshifting at a UU UUU UCC A sit e in a + 1 U mutant. There is evidence to suggest that this reduction also applies to leftward frameshifting at the same site in a - 1 U mut ant. The mutation did not increase the rate of misreading of a number of mitochondrial missense, nonsense or frameshift (of both signs) muta tions, and did not adversely affect the synthesis of wild-type mitocho ndrial gene products. It is suggested here that ribosomes bearing eith er the C-2677 mutation or its wild-type allele may behave identically during normal decoding and only differ at sites where a ribosomal stal l, by permitting non-standard decoding, differentially affects the nor mal interaction of tRNAs with the chloramphenicol resistant domain V. Chloramphenicol-resistant mutations mapping at two other sites in doma in V are described. These mutations had no effect on frameshifting.