Abstract

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 (mnm⁵s²U34) 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 mnm⁵s²U34 in tRNA. Purified tRNA^Gln_{cmnm⁵s²UUG} in the mutant strain contains a modified nucleoside similar to the novel nucleosides and the level of aminoacylation of tRNA^Gln_{cmnm⁵s²UUG} 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.