Up-promoter mutations in the positively-regulated mer promoter of Tn501.

Transcription from the mer promoter of transposon Tn501 is repressed by MerR (the product of the merR gene) in the absence of Hg2+, and activated by MerR in the presence of Hg2+. In the absence of MerR, the mer promoter has weak constitutive activity. The DNA sequence of the mer promoter shows candidate -35 and -10 sequences at the unusually high spacing of 19 base-pairs. We have selected for spontaneous mutations in the mer promoter that confer an up-promoter phenotype. Four different mutants have been isolated. Three of these are single base-pair deletions between the -10 and -35 sequences. A fourth removes the -10 sequence entirely, and places a second potential -10 sequence 17 base-pairs from the -35 sequence. None of these mutant promoters are induced by MerR in the presence of Hg2+. Two of them are repressed by MerR irrespective of the presence or absence of Hg2+. Models for the mode of action of the MerR protein are discussed in the light of these results. Our data support a mechanism in which the MerR protein in the presence of Hg2+ acts to change the conformation of DNA in the mer promoter.