Méjean V, Devedjian J C, Rives I, Alloing G, Claverys J P
Centre de Recherche de Biochimie et de Génétique Cellulaires du CNRS, Université Paul Sabatier, Toulouse, France.
Nucleic Acids Res. 1991 Oct 25;19(20):5525-31. doi: 10.1093/nar/19.20.5525.
The generalized mismatch repair system of Streptococcus pneumoniae (the Hex system) can eliminate base pair mismatches arising in heteroduplex DNA during transformation or by DNA polymerase errors during replication. Mismatch repair is most likely initiated at nicks or gaps. The present work was started to examine the hypothesis that strand discontinuities arising after removal of uracil by uracil DNA-glycosylase (Ung) can be utilised as strand discrimination signals. We show that mismatch repair efficiency is enhanced 3- to 6-fold when using uracil-containing DNA as donor in transformation. In order to assess the contribution of Ung to nascent strand discrimination for postreplication mismatch repair, we developed a positive selection procedure to isolate S. pneumoniae Ung- mutants. We succeeded in isolating Ung- mutants using this procedure based on chromosomal integration of uracil-containing hybrid DNA molecules. Cloning and characterization of the ung gene was achieved. Comparison of spontaneous mutation rates in strains either proficient or deficient in mismatch and/or uracil repair gave no support to the hypothesis that Ung plays a major role in targeting the Hex system to neosynthesized DNA strands. However Ung activity is responsible for the increased efficiency of mismatch repair observed in transformation with uracil-containing DNA. In addition Ung is involved in repair of bisulfite-treated transforming DNA.
肺炎链球菌的广义错配修复系统（Hex系统）可以消除转化过程中异源双链DNA中出现的碱基对错配，或复制过程中DNA聚合酶错误导致的碱基对错配。错配修复很可能在切口或缺口处起始。本研究旨在检验以下假设：尿嘧啶DNA糖基化酶（Ung）去除尿嘧啶后产生的链间断可作为链识别信号。我们发现，在转化过程中使用含尿嘧啶的DNA作为供体时，错配修复效率提高了3至6倍。为了评估Ung对复制后错配修复新生链识别的贡献，我们开发了一种阳性选择程序来分离肺炎链球菌Ung-突变体。我们基于含尿嘧啶的杂交DNA分子的染色体整合，利用该程序成功分离出了Ung-突变体。实现了ung基因的克隆和表征。对错配修复和/或尿嘧啶修复 proficient或缺陷的菌株的自发突变率进行比较，结果不支持Ung在将Hex系统靶向新合成DNA链中起主要作用这一假设。然而，Ung活性导致了在用含尿嘧啶的DNA进行转化时观察到的错配修复效率提高。此外，Ung参与亚硫酸氢盐处理的转化DNA的修复。
