Expression of the psl operon in Pseudomonas aeruginosa PAO1 biofilms: PslA performs an essential function in biofilm formation.

The psl gene cluster, comprising 15 cotranscribed genes from Pseudomonas aeruginosa, was recently identified as being involved in exopolysaccharide biosynthesis and biofilm formation. In this study, we investigated the regulation of the psl gene cluster and the function of the first gene in this cluster, the pslA gene. PslA shows strong similarities to UDP-glucose lipid carriers. An isogenic marker-free pslA deletion mutant of P. aeruginosa PAO1 deficient in attachment and biofilm formation was used for complementation studies. The expression of only the pslA gene, comprising a coding region of 1,437 bp, restored the biofilm-forming phenotype of the wild type, indicating that PslA is required for biofilm formation by nonmucoid P. aeruginosa. The promoter region of the psl gene cluster, which encodes PslA-PslO, was identified by rapid amplification of cDNA 5' ends. Promoter assays using transcriptional fusions to lacZ and gfp indicated a constitutive expression of the psl cluster in planktonic cells and a highly regulated and localized expression in biofilms, respectively. Expression of the psl cluster in biofilms was almost exclusively found in the centers of microcolonies, as revealed by confocal laser scanning microscopy. These data suggest that constitutive expression of the psl operon enables efficient attachment to surfaces and that regulated localized psl operon expression is required for biofilm differentiation.