Identification of a cis-acting regulatory sequence responsible for the repression of brnQ in Salmonella typhimurium

brn is the gene encoding the LIV-II transport system for branched-chain ami no acids in Salmonella typhimurium, The expression of the gene is transcrip tionally repressed by an excess of glycyl-L-leucine added to the bacterial culture. To investigate the mechanism of regulation, we constructed brnQ-la cZ translational fusions with various deletions upstream from the promoter of brnQ, and examined the effects of the deletions on the regulation. We fo und a cis-acting region, 5'-GTGTTTTA-3', for the repression of brnQ express ion, which was located 94 base pairs upstream from the transcription start site. Removal of the sequence resulted in derepression of brnQ. Two homolog ous sequences were found 45 base pairs downstream and 42 base pairs upstrea m from the sequence. We designated these sequences as O1, O2, and O3, in th e order from the sequence proximal to the promoter to that distal to the pr omoter, respectively. The gleR1 mutation, which we reported previously to b e a regulatory mutation enhancing transcription of brnQ, was a G-to-T trans version in the O1 sequence 50 base pairs upstream from the transcription st art site. Insertion of five nucleotides between O1 and O2 resulted in derep ression of brnQ. Further insertion of five nucleotides did not restore the original regulation of brnQ, indicating the importance of the proper spacin g of these sequences. We also showed that the protein product of livS, the gene responsible for regulation of the LIV-I transport system, may bind to the O2 sequence. Furthermore, LivS was shown to be an allele of Lrp based o n complementation experiments. (C) 1999 Elsevier Science B.V. All rights re served.