Identification of the genes in the Escherichia coli ileS-lsp operon. Analysis of multiple polycistronic mRNAs made in vivo.

The genes encoding isoleucyl-tRNA synthetase (ileS) and prolipoprotein signal peptidase (lsp) of Escherichia coli were previously shown to be co-transcribed (Miller, K. W., and Wu, H. C. (1987) J. Biol. Chem. 262, 389-393). However, the boundaries of this transcriptional unit have not been established. In this regard, DNA sequence determination has shown that ileS and lsp are closely flanked by four open reading frames, i.e. x-ileS-lsp-orf149-orf316-orf304. To define the boundaries of the operon, we applied Northern blotting hybridization and mRNA 5'-end mapping to analyze mRNA from a wild-type strain (SM31) and a mutant strain (SM31-2B4) that exhibits an increased expression of prolipoprotein signal peptidase. Four ileS-lsp co-transcripts were detected in RNA from the strain SM31. In addition to these four mRNAs, two new, highly abundant co-transcripts were also detected in RNA from the mutant. Based upon the determination of the 5'-ends of the mRNAs and analysis of their coding sequences, we conclude that the six mRNAs actually are comprised of three pairs of related mRNAs. The two mRNAs in a given pair have the same 5'-termini (all located upstream of or within gene x), but vary with respect to the identity of their 3'-terminal coding sequence (lsp or orf316). In conclusion, the ileS-lsp operon contains five genes, x-ileS-lsp-orf149-orf316, whose transcription probably is dependent upon promoter(s) located upstream of or within gene x. The next gene downstream, orf304, apparently does not reside in the operon.