Glucosyltransferase (GTF)-I from cariogenicStreptococcus sobrinuselon-gates thea-(1fi3)-linked glucose polymer branches on the primer dextran bound to the C-terminal glucan-binding domain. We investigated the GTF-I-catalyzed glucan synthesis reaction in the absence of the primer dextran. The time course of saccharide production during dextran-independent glu-can synthesis from sucrose was analyzed.