Characterization of lip expression in Salmonella typhimurium: analysis of lip::lac operon fusions

Strains of Salmonella typhimurium which have an auxotrophic requirement for lipoic acid were isolated by mutagenesis with the transposable element Mu dJ. The chromosomal location of these insertion mutations was determined to be at 14 map units by bacteriophage P22-mediated cotransduction. The lip gene is transcribed in the clockwise direction relative to the S. typhimurium genetic map. Strains with lip::lac operon fusions were used to characterize the transcriptional activity of the lip promoter. Transcription of the lip gene is not regulated by catabolite repression or lipoic acid concentration. The data indicate that the lip gene product is expressed constitutively at a low level.