Expression patterns of genes encoding elastase and controlling mucoidy: co‐ordinate regulation of two virulence factors in Pseudomonas aeruginosa isolates from cystic fibrosis

Transcriptional patterns of lasB and algD were compared in isogenic mucoid and non‐mucoid Pseudomonas aeruginosa isolates from cystic fibrosis patients. The lasB gene encodes elastase, a major proteolytic enzyme secreted by P. aeruginosa, while algD is required for the synthesis of alginate, an exopolysaccharide frequently overproduced by strains infecting cystic fibrosis patients. A possible coregulation at the transcriptional level of these major virulence determinants was analysed. The lasB and algD genes showed inverse levels of promoter activity. The lasB promoter was active in non‐mucoid cells and inactive in mucoid cells (in four out of five tested pairs), while the algD promoter was active in mucoid cells and silent in non‐mucoid cells in all cases. When PAO568, a model strain for the analysis of control of the alginate system, was grown under conditions promoting mucoidy, the algD promoter was activated, whereas lasB mRNA could not be detected. This effect was reversed when the cells were grown in a medium suppressing mucoidy. Insertional inactivation of algR, a member of the signal‐transduction systems regulating algD transcription, although abolishing algD expression and rendering cells non‐mucoid, did not alter the nature of the induction and repression patterns of lasB seen in the parental strain PAO568. These results suggest that the lasB gene and the alginate system are co‐ordinately regulated at a level parallel to or above the algR gene.