Pm. Giffard et Na. Jacques, DEFINITION OF A FUNDAMENTAL REPEATING UNIT IN STREPTOCOCCAL GLUCOSYLTRANSFERASE GLUCAN-BINDING REGIONS AND RELATED SEQUENCES, Journal of dental research, 73(6), 1994, pp. 1133-1141
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.