Glucansucrases of oral streptococci and Leuconostoc mesenteroides have a common pattern of structural organization and characteristically contain a domain with a series of tandem amino acid repeats in which certain residues are highly conserved, particularly aromatic amino acids and glycine. In some glucosyltransferases (GTFs) the repeat region has been identified as a glucan binding domain (GBD). Such GBDs are also found in several glucan binding proteins (GBP) of oral streptococci that do not have glucansucrase activity. Alignment of the amino acid sequences of 20 glucansucrases and GBP showed the widespread conservation of the 33-residue A repeat first identified in GtfI of Streptococcus downei. Site-directed mutagenesis of individual highly conserved residues in recombinant GBD of GtfI demonstrated the importance of the first tryptophan and the tyrosine-phenylalanine pair in the binding of dextran, as well as the essential contribution of a basic residue (arginine or lysine). A microplate binding assay was developed to measure the binding affinity of recombinant GBDs. GBD of GtfI was shown to be capable of binding glucans with predominantly alpha-1,3 or alpha-1,6 links, as well as alternating alpha-1,3 and alpha-1,6 links (alternan). Western blot experiments using biotinylated dextran or alternan as probes demonstrated a difference between the binding of streptococcal GTF and GBP and that of Leuconostoc glucansucrases. Experimental data and bioinformatics analysis showed that the A repeat motif is distinct from the 20-residue CW motif, which also has conserved aromatic amino acids and glycine and which occurs in the choline-binding proteins of Streptococcus pneumoniae and other organisms.