Molecular function and potential evolution of the biofilm-modulating blue light-signalling pathway of Escherichia coli

Escherichia coli senses blue light via the BLUF-EAL protein BluF (YcgF). The degenerate EAL domain of BluF does not have cyclic-di-GMP phosphodiesterase activity, but BluF directly antagonizes the MerR-like repressor BluR (YcgE), which leads to expression of the ycgZ-ymgABC operon and activation of the Rcs system (Tschowri et al., 2009; Genes Dev 23: 522534). While bluR, bluF and ycgZ have individual transcriptional start sites, comparative genome analysis indicates that the bluR-bluF-ycgZ-ymgAB region represents a functional unit in various enteric bacteria that is characterized by bluF alleles encoding degenerate EAL domains. Re-introducing conserved amino acids involved in phosphodiesterase activity of EAL domains did not restore enzymatic activity or c-di-GMP binding of BluF, but weakened its ability to antagonize BluR and improved a residual interaction with the BluR paralogue MlrA, which controls expression of the biofilm regulator CsgD and curli fibres. We identified the BluR binding site in the ycgZ promoter and observed that BluR also has residual affinity for the MlrA-dependent csgD promoter. Altogether, we propose that BluF evolved from a blue light-regulated PDE into a specific antagonist of a duplicate of MlrA that became BluR, which controls not only curli but various biofilm functions via the Ymg/Rcs pathway.