THE GLOBAL ACTIVATOR GACA OF PSEUDOMONAS-AERUGINOSA PAO POSITIVELY CONTROLS THE PRODUCTION OF THE AUTOINDUCER N-BUTYRYL-HOMOSERINE LACTONE AND THE FORMATION OF THE VIRULENCE FACTORS PYOCYANIN, CYANIDE, AND LIPASE

The global activator GacA, a highly conserved response regulator in Gr am-negative bacteria, is required for the production of exoenzymes and secondary metabolites in Pseudomonas spp. The gacA gene of Pseudomona s aeruginosa PAO1 was isolated and its role in cell-density-dependent gene expression was characterized. Mutational inactivation of gacA res ulted in delayed and reduced formation of the cell-density signal N-bu tyryl-L-homoserine lactone (BHL), of the cognate transcriptional activ ator RhlR (VsmR), and of the transcriptional activator LasR, which is known to positively regulate RhlR expression. Amplification of gacA on a multicopy plasmid caused precocious and enhanced production of BHL, RhlR and LasR. In parallel, the gacA gene dosage markedly influenced the BHL/RhlR-dependent formation of the cytotoxic compounds pyocyanin and cyanide and the exoenzyme lipase. However, the concentrations of a nother known cell-density signal of P. aeruginosa, N-oxododecanoyl-L-h omoserine lactone, did not always match BHL concentrations. A model ac counting for these observations places GacA function upstream of LasR and RhlR in the complex, cell-density-dependent signal-transduction pa thway regulating several exoproducts and virulence factors of P. aerug inosa via BHL.