TRANSCRIPTION OF THE GLUTAMYL-TRANSFER-RNA REDUCTASE (HEMA) GENE IN SALMONELLA-TYPHIMURIUM AND ESCHERICHIA-COLI - ROLE OF THE HEMA P1 PROMOTER AND THE ARCA GENE-PRODUCT

In Salmonella typhimurium and Escherichia coli, the hemA gene encodes the enzyme glutamyl-tRNA reductase, which catalyzes the first committe d step in the heme biosynthetic pathway. It has recently been reported that a [ac operon fusion to the hemA promoter off. coli is induced 20 -fold after starvation for heme. Induction was dependent on the transc riptional regulator ArcA, with a second transcriptional regulator, FNR , playing a negative role specifically under anaerobic conditions (S. Darie and R. P. Gunsalus, J. Bacteriol. 176:5270-5276, 1994). We have investigated the generality of this effect by examining the response t o heme starvation of a number of Inc operon fusions to the hemA promot ers of both E. coli and S. typhimurium. We confirmed that such fusions are induced during starvation of a hemA auxotroph, but the level of i nduction observed was maximally sixfold and for S. typhimurium fusions it was only two- to fourfold. Sequences required for high-level expre ssion of hemA lie within 129 bp upstream of the major (P1) promoter tr anscriptional start site. Mutants defective in the P1 promoter had gre atly reduced hemA-lac expression both in the presence and in the absen ce of ALA. Mutations in arcA had no effect on hemA-lac expression in E . coli during normal growth, although the increase in expression durin g starvation for ALA was half that seen in an arcA(+) strain. Overexpr ession of the arcA gene had no effect on hemA-lac expression. Primer e xtension analysis showed that RNA 5' ends mapping to the hemA P1 and P 2 promoters were not expressed at significantly higher levels in induc ed cultures. These results differ from those previously reported.