Cyanobacterial sigma factor controls biofilm-promoting genes through intra- and intercellular pathways

Cyanobacteria frequently constitute integral components of microbial communities known as phototrophic biofilms. These assemblages are not only widespread in various environmental contexts but also hold significant industrial relevance. Nevertheless, the governing elements responsible for cyanobacterial biofilm development have remained elusive. This study, which employs the model cyanobacterium Synechococcus elongatus PCC 7942, demonstrates that the RNA polymerase sigma factor SigF1, but not its paralog SigF2, is required for a biofilm-suppression mechanism that operates in this organism. Comprehensive transcriptome analyses identified distinct regulons under the control of each of these sigma factors. Additional data indicate that SigF1 regulates biofilm through its involvement in transcriptional induction of genes that include those for the primary pilus subunit: sigF1 inactivation both prevents pilus assembly and abrogates secretion of a biofilm inhibitor. Consequently, expression is significantly upregulated for the ebfG-operon that encodes matrix components and the genes that encode their corresponding secretion system. Thus, this study uncovers a basic regulatory component of cyanobacterial communal behavior. Elevated expression of biofilm-promoting genes in a sigF1 mutant supports an additional layer of regulation by SigF1 that operates via an intracellular mechanism.