Biosynthesis of Guanosine Diphosphate D-rhamnose and Guanosine Diphosphate D-talomethylose from Guanosine Diphosphate Alpha-d-mannose.

Strain GS, a gram-negative motile bacterium isolated from soil, synthesizes two polysaccharides. The synthesis of a uranic acid-containing polysaccharide is derepressed by high concentrations of Na2SOd, while the synthesis of the polysaccharide containing equal quantities of n-rhamnose and n-talomethylose is repressed by NazS01 (1, 2). L-F’ucose and L-rhamnose are the methylpentoses commonly found in nature. Recently, the biosynthesis of both methylpentoses has been shown to occur r&z nucleotide-linked 4-keto-6-deoxyhexose intermediates: guanosine diphosphate (GDP) 4-keto-n-rhamnose for GDP-L-fucose; thymidine diphosphate (TDP) 4-keto-6-deoxy-n-glucose for TDP-n-rhamnose) (3). This communication reports the enzymic synthesis of GDP-n-rhamnose and GDP-n-talomethylose from GDP-n-mannose by extracts of strain GS and provides evidence that at least two enzymes are involved in these conversions. In addition, data are presented which demonstrate that GDP-4-keto-n-rhamnose is the intermediate in the synthesis of GDP-n-rhamnose and GDP-n-talomethylose. Preliminary reports of this work have been published (4, 5).