Influence of the culture medium on the synthesis of α- d-glucans by Streptococcus cricetus AHT

Abstract

Three different α- d-glucosyltransferases (GTFs) were separated from culture filtrates of Streptococcus cricetus strain AHT grown in a complex. standard medium in batch culture or under defined conditions of growth in the chemostat. Two of the enzymes (GTF-s1) converted sucrose into branched soluble dextrans, and the third (GTF-I) produced a relatively linear, water-insoluble, predominantly (1→3)-linked α- d-glucan. When the organism was grown in comlex medium by the removal of the fraction of high molecular weigth, only GTF-S1 and GTF-S2 were released, and GTF-I was detected. The water-insoluble glucan fraction obtained by incubating the cell-free filtrate with sucrose contained from 17 to 25% of (1→3)-glucosidic linkages, and accounted for up to 78 and 4% of the total glucans derived from growth in standard and modified medium, respectively. The soluble glucans produced in the same reaction were fractionated with ethanol to give, from both media, two distinct dextrans comprising ( 1) a highly branched dextran similar to the S1-dextran product of GTF-S1 and ([it2 ) a dextran containing fewer branch linkages and up to 86% of (1→6)-α- d-glucosidic linkages. A GTF responsible for the synthesis of the latter dextran was not separated. The structures of the glucan fractions and the products of the separated GTF were examined by enzymic degradation and methylation analysis.