DEFINITION OF A FUNDAMENTAL REPEATING UNIT IN STREPTOCOCCAL GLUCOSYLTRANSFERASE GLUCAN-BINDING REGIONS AND RELATED SEQUENCES

The C-termini of the glucosyltransferases (Gtfs) of oral streptococci are responsible for glucan binding. These glucan-binding domains (GBDs ) are composed of a series of repeated sequences that have been classi fied into four different classes (A-D) by virtue of sequence similarit y and which, by inference, have been suggested to be of functional imp ortance. In contrast, we propose that repeat sequences evolve in respo nse to selection for an increase in the number of copies of a particul ar domain through multiple duplication events occurring at different t imes. According to this hypothesis, repeats should possess various deg rees of similarity, especially if only key residues are of functional importance. Analysis of the GBDs of the Gtfs indicated that a common f undamental repeat, designated the ''YG'' repeat, could be discerned wi thin the ''A'', ''B'', ''C'', and ''D'' repeats. Similar elements were also conserved in the ligand-binding repeats of the Clostridium diffi cile toxins and the lysins and the PspA protein of Streptococcus pneum oniae, suggesting that similar selective pressures had also been impos ed on these sequences. Analysis of the ''YG'' repeats present in the G tfJ and GtfK of Streptococcus salivarius indicated that some of the '' YG'' repeats in the GBDs of these proteins had arisen as a result of d uplication events involving a series of three sequential ''YG'' repeat s.