REGULATION OF AROL EXPRESSION BY TYRR PROTEIN AND TRP REPRESSOR IN ESCHERICHIA-COLI K-12

The promoter-operator region of the aroL gene of Escherichia coli K-12 contains three TYR R boxes and one TrpR binding site. Mutational anal ysis showed that TYR R boxes 1 and 3 are essential for TyrR-mediated r egulation of aroL expression, while a fully functional TYR R box 2 doe s not appear to be essential for regulation. Regulation mediated by th e TrpR protein required the TYR R boxes apd TrpR site to be functional and was observed in vivo only with a tyrR(+) strain. Under conditions favoring the formation of TyR hexamers, DNase I protection experiment s revealed the presence of phased hypersensitive sites, indicative bf DNA backbone strain. This suggests that TyrR-mediated repression invol ves DNA looping. Purified TrpR protein protected the putative TrpR bin ding site in the presence of tryptophan, and this protection was sligh tly enhanced in the presence of TyrR protein. This result along with t he in vivo findings implies that TyrR and TrpR are able to interact-in some way. Inserting 4 bp between TYR R box 1 and the TrpR binding sit e results in increased tyrosine repression and the abolition of the tr yptophan effect. Identification of a potential integration host factor binding site and repression studies of a himA mutant support the noti on that integration host factor binding normally exerts a negative eff ect on tyrosine-mediated repression.