Giffard P M, Jacques N A
Institute of Dental Research, United Dental Hospital, Surry Hills, NSW, Australia.
J Dent Res. 1994 Jun;73(6):1133-41. doi: 10.1177/00220345940730060201.
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 classified into four different classes (A-D) by virtue of sequence similarity and which, by inference, have been suggested to be of functional importance. In contrast, we propose that repeat sequences evolve in response to selection for an increase in the number of copies of a particular domain through multiple duplication events occurring at different times. According to this hypothesis, repeats should possess various degrees of similarity, especially if only key residues are of functional importance. Analysis of the GBDs of the Gtfs indicated that a common fundamental repeat, designated the "YG" repeat, could be discerned within the "A", "B", "C", and "D" repeats. Similar elements were also conserved in the ligand-binding repeats of the Clostridium difficile toxins and the lysins and the PspA protein of Streptococcus pneumoniae, suggesting that similar selective pressures had also been imposed on these sequences. Analysis of the "YG" repeats present in the GtfJ and GtfK of Streptococcus salivarius indicated that some of the "YG" repeats in the GBDs of these proteins had arisen as a result of duplication events involving a series of three sequential "YG" repeats.
口腔链球菌葡糖基转移酶（Gtfs）的C末端负责葡聚糖结合。这些葡聚糖结合结构域（GBDs）由一系列重复序列组成，根据序列相似性已被分为四个不同类别（A - D），据此推测这些重复序列具有功能重要性。相比之下，我们提出重复序列是为了响应通过在不同时间发生的多次重复事件来增加特定结构域拷贝数的选择而进化的。根据这一假设，重复序列应具有不同程度的相似性，特别是如果只有关键残基具有功能重要性。对Gtfs的GBDs分析表明，在“A”、“B”、“C”和“D”重复序列中可以识别出一个共同的基本重复序列，命名为“YG”重复序列。类似的元件在艰难梭菌毒素、溶素以及肺炎链球菌的PspA蛋白的配体结合重复序列中也保守存在，这表明类似的选择压力也施加于这些序列上。对唾液链球菌GtfJ和GtfK中存在的“YG”重复序列分析表明，这些蛋白质GBDs中的一些“YG”重复序列是由于涉及一系列三个连续“YG”重复序列的重复事件而产生的。
