A FleQ DNA binding consensus sequence revealed by studies of FleQ-dependent regulation of biofilm gene expression in Pseudomonas aeruginosa.

The transcription factor FleQ from Pseudomonas aeruginosa derepresses expression of genes involved in biofilm formation when intracellular levels of the second messenger cyclic diguanosine monophosphate (c-di-GMP) are high. FleQ also activates transcription of flagellar genes, and the expression of these genes is highest at low intracellular c-di-GMP. FleQ thus plays a central role in mediating the transition between planktonic and biofilm lifestyles of P. aeruginosa. Previous work showed that FleQ controls expression of the pel operon for Pel exopolysaccharide biosynthesis by converting from a repressor to an activator upon binding c-di-GMP. To explore the activity of FleQ further, we carried out DNase I footprinting at three additional biofilm gene promoters, those of psl, cdrAB, and PA2440. The expression of cdrAB, encoding a cell surface adhesin, was sufficiently responsive to FleQ to allow us to carry out in vivo promoter assays. The results showed that, similarly to our observations with the pel operon, FleQ switches from a repressor to an activator of cdrAB gene expression in response to c-di-GMP. From the footprinting data, we identified a FleQ DNA binding consensus sequence. A search for this conserved sequence in bacterial genome sequences led to the identification of FleQ binding sites in the promoters of the siaABCD operon, important for cell aggregation, and the bdlA gene, important for biofilm dispersal, in P. aeruginosa. We also identified FleQ binding sites upstream of lapA-like adhesin genes in other Pseudomonas species. IMPORTANCE The transcription factor FleQ is widely distributed in Pseudomonas species. In all species examined, it is a master regulator of flagellar gene expression. It also regulates diverse genes involved in biofilm formation in P. aeruginosa when intracellular levels of the second messenger c-di-GMP are high. Unlike flagellar genes, biofilm-associated genes are not always easy to recognize in genome sequences. Here, we identified a consensus DNA binding sequence for FleQ. This allowed us to survey Pseudomonas strains and find new genes that are likely regulated by FleQ and possibly involved in biofilm formation.