The chain length of Streptococcus pneumoniae type 3 capsular polysaccharide (cellubiuronic acid) is tightly regulated by the cellubiuronic acid synthase through an assembly process involving a catalytic motif that is potentially conserved over a wide range of related processive β-glucan synthases. Cellubiuronic acid is initiated on a lipid and is composed of alternating β-1, 3-Glc and β-1, 4-glucuronic acid (GlcUA) linkages. The entire assembly process is carried out by a polypeptide synthase thought to contain a single active site, suggesting that the donor specificity is controlled by the terminal nonreducing sugar in the acceptor subsite. Shortly after initiation, the synthase undergoes an allosteric transition accompanied by the tight binding of the nascent chain via its nonreducing oligosaccharide terminal segment to the carbohydrate acceptor recognition site. The chain length of polysaccharide assembled by recombinant synthase in Escherichia coli membranes was determined by an ejection mechanism that appeared to be a reversal of the allosteric transition of the synthase from the transitory to the fully processive state. The rates of both ejection and transition were shown to be highly sensitive to the concentration of UDP-GlcUA. As the concentration of UDP-GlcUA was increased, both the rate of synthesis and the processive turnover time increased. The product of the processive turnover time and the rate of synthesis predicted a marked increase in polysaccharide chain size (from 50 to 1150 kDa) over a relatively narrow concentration range of 1-11.5 μM UDP-GlcUA. The kinetic model chain length predictions were in close agreement with chemically determined sizes of polysaccharides synthesized at the same UDP-sugarconcentrations.Themodelindicates that translocation occurs following the addition of GlcUA to the chain terminus, whereas UDP-Glcdrives chain termination when inadequate levels of UDP-GlcUA are present. In sum, type 3 synthase appears to modulate polysaccharide chain length by functioning as a concentration-dependent kinetic timing device. © 2009 by The American Society for Biochemistry and Molecular Biology, Inc.