REGULATION OF THE SALMONELLA-TYPHIMURIUM-METF GENE BY THE METR PROTEIN

The metF gene in Escherichia coli and Salmonella typhimurium is under negative transcriptional control by the MetJ repressor. Expression of an S. typhimurium metF-lacZ gene fusion is repressed up to 10-fold by methionine addition to the growth medium in E. coli hosts encoding wil d-type MetJ repressor; this repression is not seen in metJ mutants. me tR mutations which eliminate the MetR activator protein result in two- to threefold-more-severe repression by the MetJ repressor. In a metJ metR double mutant, however, the level of metF-lacZ expression is the same as in a metJ mutant, suggesting that MetR antagonizes MetJ-mediat ed methionine repression of the metF promoter. A DNA footprint analysi s showed that MetR binds to a DNA fragment carrying the metF promoter and protects two separate regions from DNase I digestion: a 46-bp regi on from position -50 to -95 upstream of the transcription initiation s ite and a 24-bp region from about position +62 to +85 downstream of th e transcription initiation site and within the metF structural gene. N ucleotide changes in each of the MetR-binding sites away from the cons ensus sequence disrupt MetR-mediated regulation of the metF-lacZ fusio n.