Polysaccharide synthesis in Azospirillum brasilense

Abstract

The surface of bacteria consists of polysaccharides, like the exopolysaccharides and lipopolysaccharides and, as it is the case for Azospirillum brasilense, glycoproteins like glycosylated flagella. The precursors for the synthesis of polysaccharides are sugarnucleotides. Several genes, coding for enzymes involved in the synthesis and in the modification of sugarnucleotides, have been characterised is Azospirillum brasilense Sp7.

Two copies of exoB genes, encoding UDP-glucose 4′-epimerase, have been isolated. exoB1, localised on the p90 megaplasmid is responsible for 20% of UDP-glucose 4′-epimerase activity, and exoB2, localised on the chromosome is responsible for the remaining 80% of the activity. Azospirillum is the first micro-organism which has two totally functional copies of this important enzyme. exoC encodes a phosphomannomutase, involved in the synthesis of mannose-1-phosphate, a precursor for the synthesis of GDP-mannose. The upstream localised exoD encodes a GDP-mannose-dehydrogenase, which catalyzes the synthesis of GDP-mannuronic acid. The exoD gene may be cotranscribed with exoC.

In view of these data, it can be understood why mutations in some of these genes have a pleiotropic effect, whereas for others no apparent phenotype is observed.