A "gain of function" mutation in a protein mediates production of novel modified nucleosides.

The mutation sufY204 mediates suppression of a +1 frameshift mutation in the histidine operon of Salmonella enterica serovar Typhimurium and synthesis of two novel modified nucleosides in tRNA. The sufY204 mutation, which results in an amino-acid substitution in a protein, is, surprisingly, dominant over its wild-type allele and thus it is a 'gain of function' mutation. One of the new nucleosides is 5-methylaminomethyl-2-thiouridine (mnm(5)s(2)U34) modified by addition of a C10H17 side chain of unknown structure. Increased amounts of both nucleosides in tRNA are correlated to gene dosage of the sufY204 allele, to an increased efficiency of frameshift suppression, and to a decreased amount of the wobble nucleoside mnm(5)s(2)U34 in tRNA. Purified tRNA(cmnm5s2UUG)(Gln) in the mutant strain contains a modified nucleoside similar to the novel nucleosides and the level of aminoacylation of tRNA(cmnm5s2UUG)(Gln) was reduced to 26% compared to that found in the wild type (86%). The results are discussed in relation to the mechanism of reading frame maintenance and the evolution of modified nucleosides in tRNA.