Superimposition of TyrR protein-mediated regulation on osmoresponsive transcription of Escherichia coli proU in vivo

Osmotic regulation of proU expression in the enterobacteria is achieved, at least in part, by a repression mechanism involving the histone-like nucleo id protein H-NS. By the creation of binding sites for the TyrR regulator pr otein in the vicinity of the sigma(70)-controlled promoter of proU in Esche richia coli, we were able to demonstrate a superposed TyrR-mediated activat ion by L-phenylalanine (Phe), as well as repression by L-tyrosine, of proU expression in vivo. Based on the facts that pronounced activation in the pr esence of Phe was observed even at a low osmolarity and that the affinity o f binding of TyrR to its cognate sites on DNA is not affected by Phe, we ar gue that H-NS-mediated repression of proU at a low osmolarity may not invol ve it classical silencing mechanism. Our data also suggest the involvement of recruited RNA polymerase in the mechanism of antirepression in E. coli.