High osmolarity is a signal for enhanced algD transcription in mucoid and nonmucoid Pseudomonas aeruginosa strains.

Chronic lung infection with mucoid, alginate-producing strains of Pseudomonas aeruginosa is a major cause of mortality in cystic fibrosis (CF) patients. Transcriptional activation of the P. aeruginosa algD gene, which encodes GDPmannose dehydrogenase, is essential for alginate synthesis. Activation of algD is dependent on the product of the algR gene. Sequence homology between the P. aeruginosa algR gene and the Escherichia coli ompR gene, which regulates the cellular response to changes in osmolarity of the growth medium, together with the abnormally high levels of Na+ and Cl- in respiratory tract fluid in CF patients suggested that high osmolarity in the lung of the CF patient might be a signal contributing to the induction of alginate synthesis (mucoidy) in infecting P. aeruginosa. In both mucoid and nonmucoid P. aeruginosa strains (containing a functional algR gene), transcriptional activation of algD increased as the osmolarity of the culture medium increased. The increased activation of algD at high osmolarity was not in itself sufficient to induce alginate synthesis in nonmucoid strains, however, suggesting that other environmental factors are involved in full activation of the alginate genes. The targets of AlgR and OmpR, the algD promoter and the ompC and ompF promoters, respectively, were found to have appreciable sequence homology in the -60 to -110 regions. In E. coli, OmpR was capable of activating the algD promoter nearly as well as AlgR, but in both cases, activation occurred only under conditions of high osmolarity.