FIG. 1.

Overall pathway for alginate metabolism in Sphingomonas sp. strain A1. Alginate is first concentrated in a mouth-like pit on the cell surface and then incorporated into cells through the ABC transporter system consisting of AlgS, AlgM1, and AlgM2. The incorporated alginate is depolymerized by three kinds of alginate lyase (A1-I, A1-II, and A1-III) to give rise to di- and trisaccharides with an unsaturated uronyl residue at the nonreducing terminus. Alginate lyases are first synthesized as a precursor protein (Po), followed by the transformation to A1-I by excising the N-terminal peptide. A1-I is autocatalytically processed to A1-II and A1-III. The resultant oligosaccharides by the actions of A1-I, A1-II, and A1-III are degraded by oligoalginate lyase to an unsaturated monosaccharide, which is nonenzymatically converted into α-keto acid. Genes encoding proteins responsible for alginate transport and assimilation form a cluster. aly, alginate lyase (Po) gene; algS, algM1, and algM2, alginate ABC transporter genes; algQ1 and algQ2, genes with unknown functions; oal, oligoalginate lyase gene.