The growth defect in Escherichia coli deficient in peptidyl-tRNA hydrolase is due to starvation for Lys-tRNALys

Abstract

The existence of a conditional lethal temperature-sensitive mutant affecting peptidyl-tRNA hydrolase in Escherichia coli suggests that this enzyme is essential to cell survival. We report here the isolation of both chromosomal multicopy suppressors of this mutant in pth, the gene encoding the hydrolase. In one case, the cloned gene responsible for suppression is shown to be lysV, one of three genes encoding the unique lysine acceptor tRNA; 10 other cloned tRNA genes are without effect. Overexpression of lysV leading to a 2- to 3-fold increase in tRNALys concentration overcomes the shortage of peptidyl-tRNA hydrolase activity in the cell at non-permissive temperature. Conversely, in pth, supN double mutants, where the tRNALys concentration is reduced due to the conversion of lysV to an ochre suppressor (supN), the thermosensitivity of the initial pth mutant becomes accentuated. Thus, cells carrying both mutations show practically no growth at 39°C, a temperature at which the pth mutant grows almost normally. Growth of the double mutant is restored by the expression of lysV from a plasmid. These results indicate that the limitation of growth in mutants of E.coli deficient in Pth is due to the sequestration of tRNALys as peptidyl-tRNA. This is consistent with previous observations that this tRNA is particularly prone to premature dissociation from the ribosome.