PAS Domain Residues and Prosthetic Group Involved in BdlA-Dependent Dispersion Response by Pseudomonas aeruginosa Biofilms

Biofilm dispersion by Pseudomonas aeruginosa in response to environmental cues is dependent on the cytoplasmic BdlA protein harboring two sensory PAS domains and a chemoreceptor domain, TarH. The closest known and previously characterized BdlA homolog is the flavin adenine dinucleotide (FAD)-binding Aer, the redox potential sensor and aerotaxis transducer in Escherichia coli. Here, we made use of alanine replacement mutagenesis of the BdlA PAS domain residues previously demonstrated to be essential for aerotaxis in Aer to determine whether BdlA is a potential sensory protein. Five substitutions (D14A, N23A, W60A, I109A, and W182A) resulted in a null phenotype for dispersion. One protein, the BdlA protein with the G31A mutation (BdlA-G31A), transmitted a constant signal-on bias as it rendered P. aeruginosa biofilms hyperdispersive. The hyperdispersive phenotype correlated with increased interaction of BdlA-G31A with the phosphodiesterase DipA under biofilm growth conditions, resulting in increased phosphodiesterase activity and reduced biofilm biomass accumulation. We furthermore demonstrate that BdlA is a heme-binding protein. None of the BdlA protein variants analyzed led to a loss of the heme prosthetic group. The N-terminal PASa domain was identified as the heme-binding domain of BdlA, with BdlA-dependent nutrient-induced dispersion requiring the PASa domain. The findings suggest that BdlA plays a role in intracellular sensing of dispersion-inducing conditions and together with DipA forms a regulatory network that modulates an intracellular cyclic d-GMP (c-di-GMP) pool to enable dispersion.