PHYSICOCHEMICAL PROPERTIES OF EXTRACELLULAR (1-]4)-BETA-D-GLUCURONAN PRODUCED BY THE RHIZOBIUM-MELILOTI M5N1CS STRAIN DURING FERMENTATION -EVIDENCE OF DEGRADATION BY AN EXOENZYME ACTIVATED BY MG2+

The mutant Rhizobium meliloti M5N1CS (NCIMB 40472) produced a partiall y acetylated (1 --> 4)-beta-D-glucuronan during fermentation. The poly saccharide (EPS) extracted from broth during fermentation by microfilt ration and ultrafiltration (using a 100 kDa cut-off membrane) was char acterized by size exclusion chromatography, The weight-average molecul ar weight ((M) over bar(w)) of the EPS decreased from 7.5 x 10(5) to 5 X 10(5) between 27 and 75 h of fermentation. When MgSO4.7H(2)O (0.8 g l(-1) per day) was present in the medium during the same period, the (M) over bar(w) of the EPS decreased to 1.5 x 10(5). Since EPS degrada tion was detected after microfiltration of the fermentation medium sup plemented with magnesium, the (M) over bar(w) decrease of the EPS extr acted during fermentation may be explained by enzymic degradation of t he polymer activated by Mg2+ ions.