Differential regulation of two genes encoding lysyl-tRNA synthetases in Escherichia coli: lysU-constitutive mutations compensate for a lysS null mutation.

Lysyl-tRNA synthetases are synthesized in Escherichia coli from two distinct genes, lysS and lysU, which are regulated differentially. A strain which is null for lysS, the constitutive gene, was created by gene disruption (lysS1) and exhibited cold-sensitive lethality. Hence, lysS is dispensable at high temperatures. This cold sensitivity was suppressed by a multi-copy plasmid carrying lysU, the inducible gene. These data are interpreted as indicating that lysS is functionally replaceable by lysU for cell growth, and that the cold sensitivity of lysS1 is caused by insufficient expression of lysU at low temperatures. To investigate the mechanism of lysU expression, cold-resistant bypass mutations were isolated from lysS1, and named als (for abandonment of lysS). Two als mutations which were linked to lysU contain IS2 insertions upstream of the lysU promoter. They caused a 16-19-fold increase in the lysU-mRNA level. Furthermore, deletion mutations created immediately upstream of the lysU promoter restored growth of lysS1. These results suggest that transcription of lysU is negatively controlled by a cis-element located upstream of the promoter.