Mannuronan C-5 epimerases and cellular differentiation of Azotobacter vinelandii

Differentiation in Azotobacter vinelandii involves the encystment of the ve getative cell under adverse environmental circumstances and the germination of the resting cell into the vegetative state when growth conditions are s atisfactory again. Morphologically, the encystment process involves the dev elopment of a protective coat around the resting cell. This coat partly con sists of multiple layers of alginate, which is a copolymer of beta-D-mannur onic acid (M) and alpha-L-guluronic acid (G). Alginate contributes to coat rigidity by virtue of a high content of GG blocks. Such block structures ar e generated through a family of mannuronan C-5 epimerases that convert M to G after polymerization. Results from immunodetection and light microscopy, using stains that distinguish between different cyst components and types, indicate a correlation between cyst coat organization and the amount and a ppearance of mannuronan C-5 epimerases in the extracellular medium and atta ched to the cells. Specific roles of individual members of the epimerase fa mily are indicated. Calcium and magnesium ions appear to have different rol es in the structural organization of the cyst coat. Also reported is a new gene sharing strong sequence homology with parts of the epimerase-encoded R -modules. This gene is located within the epimerase gene cluster of Azotoba cter vinelandii.