Effect of magnesium ions on secretion of glucosyltransferase from Streptococcus sobrinus

Magnesium ions, which are known to inhibit the leader peptidase (signal peptidase) activity of Escherichia coli, stimulated the activities of glucosyltransferases (GTFs) in culture medium of Streptococcus sobrinus at concentrations of 1 or 5 mM. At a concentration of 50 mM, the activity of water-soluble glucan synthase was not affected and that of water-insoluble glucan synthase was slightly suppressed. The activities of both GTFs in the crude enzyme extract were totally unaffected by addition of magnesium ions. These findings indicate that magnesium ions have some effects on secretion of the GTFs from S. sobrinus, but that the mechanism of transportation of the enzymes across the plasma membrane in S. sobrinus differs completely from that of proteins secreted into the periplasmic space or assembled in the outer membrane in E. coli.