Hypomodification of the wobble base in tRNAGlu, tRNALys, and tRNAGln suppresses the temperature-sensitive phenotype caused by mutant release factor 1.

In Escherichia coli, release factor 1 (RF1) is one of two RFs that mediate termination; it specifically recognizes UAA and UAG stop codons. A mutant allele, prfA1, coding for an RF1 that causes temperature-sensitive (Ts) growth at 42 degrees C, was used to select for temperature-resistant (Ts(+)) suppressors. This study describes one such suppressor that is the result of an IS10 insertion into the cysB gene, giving a Cys(-) phenotype. CysB is a transcription factor regulating the cys regulon, mainly as an activator, which explains the Cys(-) phenotype. We have found that suppression is a consequence of the lost ability to donate sulfur to enzymes involved in the synthesis of thiolated nucleosides. From genetic analyses we conclude that it is the lack of the 5-methylaminomethyl-2-thiouridine (mnm(5)s(2)U) modification of the wobble base of tRNA(Glu), tRNA(Lys), and/or tRNA(Gln) that causes the suppressor phenotype.