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).

Readthrough and suppression of nonsense codons was compared in Escherichia coli strains with and without a miaA mutation, which confers a loss of the isopentenyladenosine modification in transfer RNA. Generally speaking, our results conform to predictions based on previous literature. In addition, we showed that the miaA mutation in strain TRPX is itself a UAA mutation. An antagonism between miaA and rpsL mutations, which confer streptomycin resistance, was also discovered. Our data further suggest that slight alterations of the translation apparatus are easily detectable by monitoring readthrough and suppression of nonsense codons. Our findings are discussed in the context of old and recent reports.

No abstract

No abstract

No abstract