A type II secretion GspC ortholog essential for biofilm self-suppression in the cyanobacterium Synechococcus elongatus

Our laboratory identified a self-suppression mechanism of biofilm development, which dictates planktonic growth of the cyanobacterium Synechococcus elongatus. Particular mutations impair this inhibition process resulting in biofilm formation. For example, inactivation of an ATPase homolog of type II secretion (T2S) systems or type IV pilus (T4P) assembly machineries yielded the biofilm forming strain, T2SEΩ. Using a barcoded random transposon library for a high throughput screen for biofilm-forming mutants, we uncovered additional components essential for the self-suppression mechanism, including Se0168. This protein, which is annotated as "conserved hypothetical" is not similar to proteins of known function. Structural modeling, however, indicated a particular domain that is similar to GspC, a periplasmic component of T2S and T4P systems. Studies of heterotrophic bacteria revealed interaction between GspC and GspD, the protein comprising the outer membrane channel. We demonstrated in vitro interaction of Se0168 with S. elongatus GspD homolog, in accordance with the structural similarity of Se0168 to GspC. Moreover, the Se0168-mutant lacks cell pili and is impaired in protein secretion, observations that further substantiate the role of Se0168 in T2S and T4P systems. Additionally, impairment of Se1068 abolished the natural competency for DNA uptake characteristic of S. elongatus. Together, this study uncovered a formerly unknown component of cyanobacterial protein secretion, pilus assembly and DNA uptake complexes of S. elongatus and demonstrated its requirement for the biofilm self-suppression mechanism operating in this organism.