The transcriptional fusion of reporter lacZ gene with cusRS regulatory promoter of cus operon of Klebsiella pneumoniae enabled us to analyze the inductive effect of copper on promoter via lacZ assay. The stimulus response curve of promoter to a range of copper metal concentrations indicated a normal sigmoidal response profile with apparent Hill coefficient 1.0. There was a positive correlation of promoter response to the increasing concentration of copper in the medium. AC(50) value of copper was calculated to be 1mM, whereas the promoter response was exponential beyond 1mM and up to 2.5mM concentration. The promoter activity did not increase exponentially in copper concentration higher than 2.5mM. The promoter PcusRS requires two chromosomally encoded regulatory proteins, CusS and CusR, in trans for maximal in vitro activation. The PcusRS regulatory promoter sequence also contained regulatory -10 and -35 boxes along with CusR binding motif. The results supported the concept of cus operon regulation as an essential mechanism for maintaining the cellular homeostasis at very high (e.g. 3mM), and even toxic copper concentrations.
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