Activator-dependent preinduction binding of sigma-70 RNA polymerase at the metal-regulated mer promoter

Expression of the Tn21 mercury-resistance (mer) locus is controlled by the merR gene product, which represses mer structural gene (merTPCAD) transcription in the absence of mercuric ion [Hg(II)] and activates it in the presence of Hg(II). In vivo DNA methylation of the mer regulatory region (merOP) shows that, with or without the inducer Hg(II), MerR strongly protects four guanine residues in a dyadic region located between the -10 and -35 hexamers of the structural gene promoter (PTPCAD). Prior to induction by Hg(II), RNA polymerase is also bound at PTPCAD; occupancy of the uninduced promoter by RNA polymerase is dependent on MerR. Methylation and permanganate footprinting demonstrate that induction by Hg(II) results in MerR/Hg(II)-dependent promoter DNA melting in the -10 region of PTPCAD and in additional DNA structural distortions within the region of dyad symmetry. Thus, MerR fosters the binding of RNA polymerase to an inactive promoter, and upon induction, MerR/Hg(II) facilitates DNA distortions suitable for efficient formation of the active transcription complex.