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Microbial Molecular Genetics and Cell Biology Research Group, School of Biological Sciences, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
Author for correspondence: N. L. Brown. Tel: +44 121 414 5467. Fax: +44 121 414 5907. e-mail: N.L.Brown@bham.ac.uk
ABSTRACT
The pco determinant of Escherichia coli plasmid pRJ1004 encodes inducible resistance to the trace element copper. The identification of two copper-dependent transcriptional initiation regions within pco that each contain a similar upstream hyphenated dyad motif is described. Deletion constructs showed that this "copper box" motif was essential for copper-inducible activity at both pco promoters, PpcoA and PpcoE. The placement of the motif differs in the two promoters, and PpcoA contains an extended -10 nonamer typical of promoters for which RNA polymerase does not bind specifically to -35 sequences. PpcoE does not contain this motif and is the more strongly expressed promoter. The transcript from PpcoA contains the pcoABCDRS genes, while PpcoE expresses only pcoE. The induction profiles for PpcoA- and PpcoE-lacZ fusions were flattened sigmoidal curves with a gradual response to increasing copper concentration. On high-copy-number plasmids, zinc was found also to induce transcription from both promoters in vivo. Both promoters showed inducible activity in the absence of pcoRS, the plasmid-borne two-component regulatory system, indicating that a second trans-acting regulatory system is present on the chromosome. The pcoR product showed repressor action in the absence of pcoS, while still allowing induction, suggesting the chromosome encoded a similar two-component system to pco. TnphoA insertion mutagenesis identified chromosomal genes which affected promoter expression, including ptsH, ptsl (sugar phosphotransferase system) and cya (adenylate cyclase). The results support that idea that pco-encoded copper resistance is an auxiliary mechanism for handling copper, the regulation of which is integrated with the chromosomal regulation of cellular copper metabolism.
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