Transcription and regulation of expression of the Escherichia coli methionyl-tRNA synthetase gene.

The DNA sequence and transcriptional organization around the Escherichia coli methionyl-tRNA synthetase gene, metG, were resolved. This gene can be transcribed in vivo and in vitro from two distinct promoters separated by 510 nucleotides. The upstream promoter is located within the coding sequence of a divergent gene expressing a protein of Mr 39 kDa of unknown function. Transcription originating from this upstream promoter is attenuated by a Rho-independent terminator before entering the structural gene. This leader RNA contains several potentially stable secondary structures, one of which shows striking similarity to tRNA(Met), but no methionine-rich coding sequence. The regulation of metG expression was investigated by means of fusions to the lacZ gene. Transcription of a metG::lacZ fusion is induced in a metG mutant and, reciprocally, repression is observed in a methionyl-tRNA synthetase overproducing strain. A model of metG expression control is proposed.