THE FUNCTION OF THE UPSTREAM REGION OF THE SIGMA(54)-DEPENDENT KLEBSIELLA-PNEUMONIAE NIFL PROMOTER IS SENSITIVE TO DNA SUPERCOILING

The positive control protein NTRC activates transcription from the sig ma54-dependent nifL and glnAp2 promoters of Klebsiella pneumoniae by b inding to upstream enhancer-like sequences and contacting downstream b ound sigma54 -RNA polymerase via looping of the intervening DNA. In co ntrast to the glnAp2 promoter, the activity of the nifL promoter is ve ry sensitive to changes in DNA supercoiling both in vivo and in vitro. We have shown previously that the downstream elements of the nifL pro moter are involved in the supercoiling response. In this study we find that the upstream region of nifL influences the supercoiling response of a hybrid nifL-glnAp2 promoter both in vivo and in vitro, demonstra ting that the nifL upstream region also confers supercoiling sensitivi ty. DNA supercoiling did not appear to influence binding of NTRC to it s sites in the nifL upstream region, suggesting that another function of this region, most probably DNA loop formation, is sensitive to chan ges in DNA topology.