KINETIC AND METABOLIC EFFECTS OF NITROGEN, MAGNESIUM AND SULFUR RESTRICTION IN XANTHOMONAS-CAMPESTRIS BATCH CULTURES

By reducing the concentration of nitrogen (from 5.0 to 2.5 mmol l-1), batch cultures of Xanthomonas campestris induced the enzyme UDP-glucos e dehydrogenase and stimulated the Entner-Doudoroff pathway enzyme glu cose-6-P dehydrogenase. The surplus energy generation was directed to xanthan biosynthesis resulting in a 10% polysaccharide increase. The n itrogen restriction led to a higher consumption of nitrogen (93%) wher eas glucose consumption did not surpass 75% utilization. Low concentra tions of both magnesium and sulphur exerted a negative effect on xanth an formation. Both restrictions reduced the phosphomannose isomerase e nzyme activity by 10-fold turning the mannose transference presumably into the rate-limiting step for xanthan biosynthesis. Conversely, the rate of synthesis of glucuronic acid residues did not affect the rate of xanthan biosynthesis. Polysaccharide synthesis in magnesium and sul phur cultures was negatively affected in comparison with cell formatio n as the cell volumetric production rate increased from 0.037 to 0.091 g l-1 h-1 and the xanthan volumetric production rate dropped from 0.1 33 g l-1 h-1 to the minimum obtained at 0.083 g l-1 h-1. The efficienc y of the carbon substrate conversion was also greatly changed.