OSMOPROTECTANTS AND PHOSPHATE REGULATE EXPRESSION OF PHOSPHOLIPASE-C IN PSEUDOMONAS-AERUGINOSA

Phospholipase C has been increasingly recognized as a significant virulence determinant in the pathogenesis of Gram-negative and Gram-positive infections. Pseudomonas aeruginosa carries two, non-tandem genes encoding phospholipase C (PLC) activity. One PLC (PLC-H) haemolyses human and sheep erythrocytes while the other is not haemolytic for these kinds of red blood cells. It was previously determined that the synthesis of both PLCs is regulated by inorganic phosphate (Pi), but little else was known regarding the regulation of these potentially important virulence determinants of P. aeruginosa. In this report, data are presented demonstrating that both PLC genes are regulated at the transcriptional level by Pi and by a P. aeruginosa homologue of the positive regulator of genes in the Pi regulon of Escherichia coli, i.e. PhoB. In addition to Pi, it is also shown in this report that the synthesis of both PLC-H and PLC-N is induced by compounds which are not only derived from the substrate product of both enzymes, i.e. phosphorylcholine, but are also known osmoprotectants in eukaryotic and prokaryotic cells. The osmoprotective derivatives of phosphorylcholine which induce the synthesis of PLC in P. aeruginosa include choline, glycine betaine, and dimethylglycine, but not sarcosine (monomethylglycine) or glycine. By constructing mutants which are deficient in the production of each separate PLC and in the production of PhoB it was determined that induction of PLC-H by the osmoprotective compounds is independent of Pi concentration and PhoB, while induction of PLC-N by these compounds requires Pi-deficient conditions and PhoB. It was further demonstrated that while an ionic osmolyte, such as NaCl, inhibited production of both enzymes in Pi-deficient conditions, the presence of osmoprotective compounds, such as choline, restored synthesis of PLC when P. aeruginosa was grown in 0.5 M NaCl. In contrast to the inhibition of PLC production in 0.5 M NaCl in Pi-deficient conditions, PLC production in an equivalent osmotic pressure generated by the neutral osmolyte sucrose was not inhibited, and choline markedly induced production of PLC in this ionically neutral, high osmotic environment. These observations are especially relevant to the pathogenic potential of PLC in the dehydrating (hence high osmotic) conditions of the lungs of cystic fibrosis patients.