The sll1951 gene encodes the surface layer protein of Synechocystis sp. strain PCC 6803

Sll1951 is the surface layer (S-layer) protein of the cyanobacterium Synechocystis sp. strain PCC 6803. This large, hemolysin-like protein was found in the supernatant of a strain that was deficient in S-layer attachment. An sll1951 deletion mutation was introduced into Synechocystis and was easily segregated to homozygosity under laboratory conditions. By thin-section and negative-stain transmission electron microscopy, a ~30-nm-wide S-layer lattice covering the cell surface was readily visible in wild-type cells but was absent in the Δsll1951 strain. Instead, the Δsll1951 strain displayed a smooth lipopolysaccharide surface as its most peripheral layer. In the presence of chaotropic agents, the wild type released a large (>150-kDa) protein into the medium that was identified as Sll1951 by mass spectrometry of trypsin fragments; this protein was missing in the Δsll1951 strain. In addition, Sll1951 was prominent in crude extracts of the wild type, indicating that it is an abundant protein. The carotenoid composition of the cell wall fraction of the Δsll1951 strain was similar to that of the wild type, suggesting that the S-layer does not contribute to carotenoid binding. Although the photoautotrophic growth rate of the Δsll1951 strain was similar to that of the wild-type strain, the viability of the Δsll1951 strain was reduced upon exposure to lysozyme treatment and hypo-osmotic stress, indicating a contribution of the S-layer to the integrity of the Synechocystis cell wall. This work identifies the S-layer protein in Synechocystis and shows that, at least under laboratory conditions, this very abundant, large protein has a supportive but not a critical role in the function of the cyanobacterium.