Abstract
Lysyl-tRNA synthetases [L-lysine:tRNA(Lys) ligase (AMP-forming), EC 6.1.1.6] are synthesized from two distinct genes in Escherichia coli, lysS (constitutively) and lysU (inducibly), but neither the physiological significance nor the mechanism of differential regulation of these two genes is understood. We have constructed a null mutation of lysS that causes cold-sensitive lethality and then used this mutant to acquire and characterize several bypass mutations called als (abandonment of lysS). Cold-resistant survivors were isolated either spontaneously or by transposon-mediated disruption, and all caused derepression of lysU transcription. One class of als mutations is linked to lysU and presumably affects the cis regulatory element. Mutations of the other class map within the lrp gene, which encodes the leucine-responsive regulatory protein (Lrp). A lysU-lacZ gene fusion study revealed that lysU is susceptible to thermal regulation in the absence of lrp and that a small mRNA region immediately downstream of the initiation codon is required for potentially high-level expression. These results suggest that lysU is part of the leucine regulon and is both negatively controlled by Lrp and positively regulated by a potential translational enhancer sequence. This sequence is similar to that of the "downstream box" complementary to nucleotides 1469-1483 of 16S rRNA, which can be universally found in tRNA synthetase genes of E. coli.