Physiological analysis of the role of truB in Escherichia coli: A role for tRNA modification in extreme temperature resistance

The truB gene of Escherichia coli encodes the pseudouridine-55 (ψ55) synthase and is responsible for modifying all tRNA molecules in the cell at the U55 position. A truB null mutant grew normally on all growth media tested, but exhibited a competitive disadvantage in extended co-culture with its wild-type progenitor. The mutant phenotype could be complemented by both the cloned truB gene and by a D48C, catalytically inactive allele of truB. The truB mutant also exhibited a defect in survival of rapid transfer from 37 to 50 °C. This mutant phenotype could be complemented by the cloned truB gene but not by a D48C, catalytically inactive allele of truB. The temperature sensitivity of truB mutants could be enhanced by combination with a mutation in the trmA gene, encoding an m⁵U-methyltransferase, modifying the universal U54 tRNA nucleoside, but not by mutations in trmH, encoding the enzyme catalysing the formation of Gm18. The truB mutant proteome contained altered levels of intermediates involved in biogenesis of the outer-membrane proteins OmpA and OmpX. The truB mutation also reduced the basal expression from two σ^E promoters, degP and rpoHP3. Three novel aspects to the phenotype of truB mutants were identified. Importantly the data support the hypothesis that TruB-effected ψ55 modification of tRNA is not essential, but contributes to thermal stress tolerance in E. coli, possibly by optimizing the stability of the tRNA population at high temperatures.