Myxoxanthophyll Is Required for Normal Cell Wall Structure and Thylakoid Organization in the Cyanobacterium Synechocystis sp. Strain PCC 6803

Abstract

Myxoxanthophyll is a carotenoid glycoside in cyanobacteria that is of unknown biological significance. The sugar moiety of myxoxanthophyll in Synechocystis sp. strain PCC 6803 was identified as dimethyl fucose. The open reading frame sll1213 encoding a fucose synthetase orthologue was deleted to probe the role of fucose and to determine the biological significance of myxoxanthophyll in Synechocystis sp. strain PCC 6803. Upon deletion of sll1213, a pleiotropic phenotype was obtained: when propagated at 0.5 mol photons m ؊2 s ؊1 , photomixotrophic growth of cells lacking sll1213 was poor. When grown at 40 mol photons m ؊2 s ؊1 , growth was comparable to that of the wild type, but cells showed a severe reduction in or loss of the glycocalyx (S-layer). As a consequence, cells aggregated in liquid as well as on plates. At both light intensities, new carotenoid glycosides accumulated, but myxoxanthophyll was absent. New carotenoid glycosides may be a consequence of less-specific glycosylation reactions that gained prominence upon the disappearance of the native sugar moiety (fucose) of myxoxanthophyll. In the mutant, the N-storage compound cyanophycin accumulated, and the organization of thylakoid membranes was altered. Altered cell wall structure and thylakoid membrane organization and increased cyanophycin accumulation were also observed for ⌬slr0940K, a strain lacking -carotene desaturase and thereby all carotenoids but retaining fucose. Therefore, lack of myxoxanthophyll and not simply of fucose results in most of the phenotypic effects described here. It is concluded that myxoxanthophyll contributes significantly to the vigor of cyanobacteria, as it stabilizes thylakoid membranes and is critical for S-layer formation.

The sll1951 Gene Encodes the Surface Layer Protein of Synechocystis sp. Strain PCC 6803

“…We detected Sll1951, a large protein that resembles hemolysin and that has been characterized before (22,23), in the growth medium of the ⌬sll1213 mutant strain of Synechocystis (data not shown); this mutant is unable to synthesize fucose and appears to lack the S-layer (24), thus laying the foundation for the present study.…”

“…Transmission electron microscopy of cells fixed by high-pressure freezing was performed on photoautotrophically grown strains of Synechocystis wild type and the ⌬sll1951 strain, as well as a ⌬sll1213 strain, lacking myxoxanthophyll due to deletion of the fucose synthetase gene (24). Both the ⌬sll1951 and ⌬sll1213 strains showed an altered cell wall relative to the wild type: wild-type cells were surrounded by a surface layer spanning a distance of approximately 30 nm from the outer membrane (OM), whereas this layer was absent in both deletion strains (Fig.…”

“…Our interest in Sll1951 was based initially on the analysis of a Synechocystis ⌬sll1213 mutant (24), which shed significant amounts of Sll1951 into the medium, and our interest was furthered by the characterization of the ⌬sll1951 deletion strain, which lacks the ability to synthesize the S-layer protein.…”

“…8). This carotenoid may be myxol (24), but it was not characterized further, as it was not affected by the lack of Sll1951. In contrast to the cell wall fraction, the yellow cytoplasmic membrane fraction was of lowest density and barely penetrated the sucrose on top of the gradient.…”

“…As the cell wall may contain carotenoids (24,38), the carotenoid content of both strains was analyzed with a focus on the cell periphery. The distribution and concentration of carotenoids in the mutant remained largely unchanged (Fig.…”