Regulation of cyanobacterial biofilm formation: Modulation of a sigma factor essential for biofilm self-suppression

The switch between planktonic and sessile growth is a critical decision in bacterial developmental that is tightly regulated. We recently revealed that sigma factor of RNA polymerase, SigF1, is essential for the biofilm self-suppression mechanism that operates in the unicellular cyanobacterium Synechococcus elongatus PCC 7942. Additionally, we have shown that the type IV pilus assembly complex is necessary for the biofilm inhibitory process. To gain insight into regulation of this sigma factor we identified its cellular interactors. These data suggest involvement of a Ser/Thr phosphatase in SigF1 regulation and biofilm formation. Moreover, transcription analysis by qRT-PCR supported involvement of the Ser/Thr phosphatase in transcription induction of the ebfG-operon, which encodes components of the biofilm matrix and is essential for biofilm formation. Furthermore, phospho-proteome analysis indicated phosphorylation of Thr237 located within the sigma-4 domain present in SigF1. Together, the data suggest a role for phosphorylated SigF1 in repression of the ebfG-operon. Additionally, we revealed interaction of SigF1 with the type IV pilus assembly complex. This interaction suggests sequestration of the sigma factor by the type IV pilus assembly apparatus thereby assigning to this complex a role as an anti-sigma factor.