el Karoui M, Ehrlich D, Gruss A
Institut National de la Recherche Agronomique, Jouy en Josas, France.
Proc Natl Acad Sci U S A. 1998 Jan 20;95(2):626-31. doi: 10.1073/pnas.95.2.626.
Studies of RecBCD-Chi interactions in Escherichia coli have served as a model to understand recombination events in bacteria. However, the existence of similar interactions has not been demonstrated in bacteria unrelated to E. coli. We developed an in vivo model to examine components of dsDNA break repair in various microorganisms. Here, we identify the major exonuclease in Lactococcus lactis, a Gram-positive organism evolutionarily distant from E. coli, and provide evidence for exonuclease-Chi interactions. Insertional mutants of L. lactis, screened as exonuclease-deficient, affected a single locus and resulted in UV sensitivity and recombination deficiency. The cloned lactococcal genes (called rexAB) restored UV resistance, recombination proficiency, and the capacity to degrade linear DNA, to an E. coli recBCD mutant. In this context, DNA degradation is specifically blocked by the putative lactococcal Chi site (5'-GCGCGTG-3'), but not by the E. coli Chi (5'-GCTGGTGG-3') site. RexAB-mediated recombination was shown to be stimulated approximately 27-fold by lactococcal Chi. Our results reveal that RexAB fulfills the biological roles of RecBCD and indicate that its activity is modulated by a short DNA sequence. We speculate that exonuclease/recombinase enzymes whose activities are modulated by short DNA sequences are widespread among bacteria.
对大肠杆菌中RecBCD-Chi相互作用的研究已成为理解细菌重组事件的一个模型。然而，在与大肠杆菌无关的细菌中尚未证实存在类似的相互作用。我们开发了一种体内模型来研究各种微生物中双链DNA断裂修复的组成部分。在此，我们鉴定了乳酸乳球菌（一种在进化上与大肠杆菌距离较远的革兰氏阳性菌）中的主要核酸外切酶，并提供了核酸外切酶与Chi相互作用的证据。作为核酸外切酶缺陷型筛选出的乳酸乳球菌插入突变体影响单个位点，并导致紫外线敏感性和重组缺陷。克隆的乳球菌基因（称为rexAB）恢复了大肠杆菌recBCD突变体的紫外线抗性、重组能力和降解线性DNA的能力。在这种情况下，DNA降解被假定的乳球菌Chi位点（5'-GCGCGTG-3'）特异性阻断，但不被大肠杆菌Chi位点（5'-GCTGGTGG-3'）阻断。结果表明，乳球菌Chi可使RexAB介导的重组增强约27倍。我们的结果表明，RexAB履行了RecBCD的生物学功能，并表明其活性受短DNA序列调控。我们推测，其活性受短DNA序列调控的核酸外切酶/重组酶在细菌中广泛存在。
