The alternative sigma factor SigF1 is essential for biofilm self-suppression in Synechococcus elongatus PCC7942

The mechanisms underlying cyanobacterial biofilm regulation started emerging only in recent years in spite of the environmental prevalence and the economic burden associated with these assemblages. We previously revealed a biofilm self-suppression mechanism that operates in Synechococcus elongatus PCC7942 and which requires the type IV pilus (T4P) assembly apparatus. This complex is involved in deposition of a biofilm inhibitor to the extracellular milieu, which in turn suppresses expression of the EbfG-operon that comprises genes enabling biofilm formation. Our recent studies demonstrate that the alternative sigma factor, SigF1, is essential for the biofilm inhibitory process. The SigF1-inactivated strain (SigFΩ) forms robust biofilms when grown in fresh medium, however, growth in conditioned medium from a wild-type culture completely suppresses biofilm formation. These data suggest that SigFΩ is impaired in synthesis or secretion of the inhibitor but is capable of responding to it once provided in conditioned medium. Additionally, SigFΩ lacks cell pili and strongly upregulates the EbfG-operon. Comparative transcriptome and exo-proteome analyses of SigFΩ and PilBΩ, which is inactivated in the gene encoding the assembly ATPase of the T4P complex, indicate much higher expression and secretion of EbfG proteins in SigFΩ compared with PilBΩ. Additionally, transcription of pilA that encodes the pilus subunit is substantially downregulated in SigFΩ. Together, the data support regulation of biofilm formation in S. elongatus by two pathways: Direct repression of the EbfG-operon by SigF1 as well as its involvement in secretion of the biofilm inhibitor via transcription activation of pilA.