Alteration in the glycan pattern of pilin in a nonmotile mutant of Synechocystis sp PCC 6803

Pilus-mediated motility is essential for the optimization of photosynthesis and environmental adaptation in the cyanobacterium Synechocystis sp. PCC 6803 (Syn6803). To identify the genes required for pilus-mediated motility in Syn6803, we applied a forward genetic approach using a Th5 mutant library and reverse genetics using interposon mutagenesis. One of the identified genes, sll0899, bears sequence similarity to acyltransferases and nucleotidyltransferases [1]. The sll0899 gene product is not involved in the transcription or translation of pilA1, which encodes pilin, the major component of pili. Instead, the sll0899::Cm-r mutant produced pilins with increased molecular mass, suggesting the existence of different PTMs. Using MS, we found that the wild-type (WT) and mutant pilins were glycosylated between amino acids 67 and 75. Analyses by quantitative MS and high-pH anion exchange chromatography (HPAEC) revealed that the glycan in WT pilin is composed of xylose and fucose, whereas an additional sugar, rhamnose, was found in the glycan of sll0899::Cm-r Our findings suggest that an alteration in the O-linked glycan of pilin is responsible for the loss of pilus-mediated motility in sll0899::Cm-r.