Mechanism of repression of the aroP P2 promoter by the TyrR protein of Escherichia coli

Previously, we have shown that expression of the Escherichia coli aroP P2 p romoter is partially repressed by the TyrR protein alone and strongly repre ssed by the TyrR protein in the presence of the coeffector tyrosine or phen ylalanine (P. Wang, J. Yang, and A. J. Pittard, J. Bacteriol. 179:4206-4212 , 1997). Here we present in vitro results showing that the TyrR protein and RNA polymerase can bind simultaneously to the aroP P2 promoter. In the pre sence of tyrosine, the TyrR protein inhibits open complex formation at the P2 promoter, whereas in the absence of any coeffector or in the presence of phenylalanine, the TyrR protein inhibits a step(s) following the formation of open complexes. We also present mutational evidence which implicates th e N-terminal domain of the TyrR protein in the repression of P2 expression. The TyrR binding site of aroP, which includes one weak and one strong TyrR box, is located 5 bp downstream of the transcription start site of P2. Res ults from a mutational analysis show that the strong box (which is located more closely to the P2 promoter), but not the weak box, plays a critical ro le in P2 repression.