Cyanobacterial biofilms, which are highly ubiquitous and inhabit diverse niches, are often associated with biological fouling and cause severe economic loss. Information on the molecular mechanisms underlying biofilm formation in cyanobacteria is scarce contrary to the wealth of knowledge on mechanisms involved in biofilm formation, maintenance and dispersal in heterotrophs. Synechococcus elongatus PCC7942 grows planktonically under laboratory conditions. Intriguingly, inactivation of a gene encoding homolog of the ATPase subunit of type II protein secretion/type IV pilus assembly systems results in biofilm formation. The mutant is impaired in protein secretion and in contrast to the wild type, is nonpilliated. The cell-free extracellular fluids (conditioned medium) from a wild type culture represses biofilm formation by the mutant. This suggested that the planktonic nature of the wild type strain is a result of a self-inhibition mechanism, which depends on the deposition of a factor to the extracellular milieu. We also identified genes that are essential for biofilm formation. Bioinformatics analyses suggest that four of these genes encode secreted small proteins. Additionally, we identified, several genes encoding components putatively involved in secretion and processing of these small proteins. The genes encoding the putative secreted proteins are highly transcribed in the biofilm-forming mutant, whereas, conditioned medium from a wild type culture, which inhibits biofilm formation, decreases the abundance of these transcripts. Taken together, we suggest that mechanisms that promote biofilm development are governed by an extracellular inhibitory factor. The role of intercellular communication in regulation of biofilm development will be discussed.
