THE PROTEIN HU CAN DISPLACE THE LEXA REPRESSOR FROM ITS DNA-BINDING SITES

The major bacterial histone-like protein HU is a small, basic, dimeric protein composed of two closely related subunits. HU is involved in s everal processes in the bacterial cell such as the initiation of repli cation, transposition, gene inversion and cell division. It has been s uggested that HU could introduce structural changes to the DNA which w ould facilitate or inhibit the binding of regulatory proteins to their specific sites. In this study we investigated the effect of HU on the binding of LexA protein, the regulator of SOS functions, to three of its specific binding sites. We show that HU can displace LexA from its binding sites on the operators of the lexA, recA and sfiA genes. The lexA operator was the most sensitive while the higher affinity sfiA op erator was the least sensitive. Since HU, like its homologue IHF, prob ably binds DNA in the minor groove we tested the effect of distamycin, a drug which binds to the minor groove, on LexA binding. Like HU, thi s drug disrupted LexA-operator complexes. These results suggest that d istortion of the minor groove of the lexA operators excludes the bindi ng of the repressor to the major groove.