The role of an alternative sigma factor in motility and pilus formation in the cyanobacterium Synechocystis sp. strain PCC6803

Disruption of a gene for an alternative sigma factor, designated sigF, in the freshwater, unicellular cyanobacterium Synechocystis sp, strain PCC6803 resulted in a pleiotropic phenotype. Most notably, this mutant lost phototactic movement with a concomitant loss of pill, which are abundant on the surface of wild-type cells. The sigF mutant also secreted both high levels of yellow-brown and UV. absorbing pigments and a polypeptide that is similar to a large family of extracellular proteins that includes the hemolysins. Furthermore, the sigF mutant had a dramatically reduced level of the transcript from two tandemly arranged bill genes (sll1694 and sll1695), which encode major structural components of type IV pill, Inactivation of these piL-1 genes eliminated phototactic movement, though some pill were still present in this strain. Together, these results demonstrate that SigF plays a critical role in motility via the control of pill formation and is also likely to regulate other features of the cell surface. Furthermore, the data provide evidence that type IV pill are required for phototactic movement in certain cyanobacteria and suggest that different populations of pill present on the Synechocystis cell surface may perform different functions.