NovelE. colimutants deficient in blosynthesis of 5-methylamlnomethyl-2-thiouridine

Novel E. coli mutants deficient in biosynthesis of 5- methylaminomethyl -2-thiouridine were isolated based on a phenotype of reduced readthrough at UAG codons. They define 2 new loci trmE and trmF , near 83' on the E. coli map. These mutants are different from strains carrying trmC mutations, which are known to confer a methylation deficiency in biosynthesis of 5- methylaminomethyl -2-thiouridine. tRNA from mutants carrying trmE or trmF mutations was shown to carry 2-thiouridine instead of 5- methylaminomethyl -2-thiouridine. This deficiency affects the triplet binding properties of the mutant tRNA. Our results suggest that the 5- methylaminomethyl group stabilizes the basepairing of this modified nucleotide with G, most likely through direct interaction with the ribosomal binding site(s).