PCHR, A REGULATOR OF FERRIPYOCHELIN RECEPTOR GENE (FPTA) EXPRESSION IN PSEUDOMONAS-AERUGINOSA, FUNCTIONS BOTH AS AN ACTIVATOR AND AS A REPRESSOR

The product of the pchR gene, an AraC-like regulatory protein, is requ ired for production of the FptA ferric pyochelin receptor in response to iron limitation and pyochelin (D. E. Heinrichs and K. Poole, J. Bac teriol, 175:5882-5889, 1993), The influence of iron, pyochelin, PchR, and FptA on fptA and pchR gene expression was assessed with fptA-lacZ and pchR-lacZ transcriptional fusions, As was expected, the expression of fptA decreased dramatically following the inactivation of pchR by the insertion of an Omega Hg cartridge, although the effect (>10-fold) was not as dramatic as that of pyochelin deficiency, which obviated f ptA gene expression, Insertional inactivation of pchR in a pyochelin-d eficient (Pch(-)) background restored fptA expression to levels observ ed in the pyochelin-producing (Pch(+)) PchR(-) strain, suggesting that PchR represses fptA expression in the absence of pyochelin, Consisten t with this, the cloned gene caused a five-fold decrease in the expres sion of the fptA-lacZ fusion in Escherichia coil, pchR gene expression was inducible by iron limitation, a result in agreement with the prev ious identification of a Fur box upstream of the gene, although the ma gnitude of the induction was less than that observed for fptA in respo nse to iron limitation. Expression of pchR was effectively absent in a pyochelin-deficient strain, and insertional inactivation of pchR in a Pch(+) or Pch(-) background caused an increase in pchR gene expressio n. PchR, thus, negatively regulates its own expression, Two related he ptameric sequences, CGAGGAA and CGTGGAT, were identified upstream of t he putative -35 region of both fptA and pchR and may function as a bin ding site for PchR. Insertional inactivation of fptA caused a marked d ecrease in fptA expression in a Pch(+) background and obviated the app arent repression of fptA expression in a Pch(-) background, reminiscen t of the effect of a pchR mutation, The fptA mutant did not, however, exhibit a defect in pchR expression, Interestingly, fptA mutants were unable to grow in the presence of pyochelin, suggesting that FptA is t he sole outer membrane receptor for ferric pyochelin. These data indic ate that PchR functions as both an activator and a repressor in contro lling the expression of fptA and pchR. The involvement of FptA in this control is unclear, although it may be important in mediating the pyo chelin effect on fptA expression, possibly by modulating PchR activity .