CHARACTERIZATION OF A TEMPERATURE-SENSITIVE ESCHERICHIA-COLI MUTANT AND REVERTANTS WITH ALTERED SERYL-TRANSFER-RNA SYNTHETASE-ACTIVITY

A mutation in the structural gene coding for seryl-tRNA synthetase in temperature-sensitive Escherichia coli K28 has been reported to alter the level of enzyme expression at high temperature (R. J. Hill and W. Konigsberg, J. Bacteriol, 141:1163-1169, 1980). We identified this mut ation as a C-->T transition in the first base of codon 386, resulting in a replacement of histidine by tyrosine, The steady-state levels of serS mRNA in K28 and in the wild-type strains are very similar, Pulse- chase labeling experiments show a difference in protein stability, but not one important enough to account for the temperature sensitivity o f K28. The main reason for the temperature sensitivity of K28 appears to be the low level of specific activity of the mutant synthetase at n onpermissive temperature, not a decreased expression level, Spontaneou s temperature-resistant revertants were selected which were found to h ave about a fivefold-higher level of SerRS than the K28 strain. We ide ntified the mutation responsible for the reversion as being upstream f rom the -10 sequence in the promoter region. The steady-state levels o f serS mRNA in the revertants are significantly higher than that in th e parental strain.