Wang J, Shoemaker N B, Wang G R, Salyers A A
Department of Microbiology, University of Illinois, Urbana 61801, USA.
J Bacteriol. 2000 Jun;182(12):3559-71. doi: 10.1128/JB.182.12.3559-3571.2000.
The mobilizable Bacteroides element NBU2 (11 kbp) was found originally in two Bacteroides clinical isolates, Bacteroides fragilis ERL and B. thetaiotaomicron DOT. At first, NBU2 appeared to be very similar to another mobilizable Bacteroides element, NBU1, in a 2.5-kbp internal region, but further examination of the full DNA sequence of NBU2 now reveals that the region of near identity between NBU1 and NBU2 is limited to this small region and that, outside this region, there is little sequence similarity between the two elements. The integrase gene of NBU2, intN2, was located at one end of the element. This gene was necessary and sufficient for the integration of NBU2. The integrase of NBU2 has the conserved amino acids (R-H-R-Y) in the C-terminal end that are found in members of the lambda family of site-specific integrases. This was also the only region in which the NBU1 and NBU2 integrases shared any similarity (28% amino acid sequence identity and 49% sequence similarity). Integration of NBU2 was site specific in Bacteroides species. Integration occurred in two primary sites in B. thetaiotaomicron. Both of these sites were located in the 3' end of a serine-tRNA gene NBU2 also integrated in Escherichia coli, but integration was much less site specific than in B. thetaiotaomicron. Analysis of the sequence of NBU2 revealed two potential antibiotic resistance genes. The amino acid sequences of the putative proteins encoded by these genes had similarity to resistances found in gram-positive bacteria. Only one of these genes was expressed in B. thetaiotaomicron, the homolog of linA, a lincomycin resistance gene from Staphylococcus aureus. To determine how widespread elements related to NBU1 and NBU2 are in Bacteroides species, we screened 291 Bacteroides strains. Elements with some sequence similarity to NBU2 and NBU1 were widespread in Bacteroides strains, and the presence of linA(N) in Bacteroides strains was highly correlated with the presence of NBU2, suggesting that NBU2 has been responsible for the spread of this gene among Bacteroides strains. Our results suggest that the NBU-related elements form a large and heterogeneous family, whose members have similar integration mechanisms but have different target sites and differ in whether they carry resistance genes.
可移动的拟杆菌元件NBU2（11千碱基对）最初是在两种拟杆菌临床分离株中发现的，即脆弱拟杆菌ERL和多形拟杆菌DOT。起初，NBU2在一个2.5千碱基对的内部区域似乎与另一个可移动的拟杆菌元件NBU1非常相似，但现在对NBU2完整DNA序列的进一步研究表明，NBU1和NBU2之间的近乎相同的区域仅限于这个小区域，并且在这个区域之外，这两个元件之间几乎没有序列相似性。NBU2的整合酶基因intN2位于该元件的一端。该基因对于NBU2的整合是必要且充分的。NBU2的整合酶在C末端具有在λ家族的位点特异性整合酶成员中发现的保守氨基酸（R-H-R-Y）。这也是NBU1和NBU2整合酶唯一共享任何相似性的区域（氨基酸序列同一性为28％，序列相似性为49％）。NBU2在拟杆菌属物种中的整合是位点特异性的。在多形拟杆菌中有两个主要位点发生整合。这两个位点都位于丝氨酸-tRNA基因的3'末端。NBU2也整合到大肠杆菌中，但整合的位点特异性比在多形拟杆菌中低得多。对NBU2序列的分析揭示了两个潜在的抗生素抗性基因。这些基因编码的推定蛋白质的氨基酸序列与在革兰氏阳性细菌中发现的抗性具有相似性。这些基因中只有一个在多形拟杆菌中表达，即来自金黄色葡萄球菌的林可霉素抗性基因linA的同源物。为了确定与NBU1和NBU2相关的元件在拟杆菌属物种中的广泛程度，我们筛选了291株拟杆菌菌株。与NBU2和NBU1具有一些序列相似性的元件在拟杆菌菌株中广泛存在，并且拟杆菌菌株中linA（N）的存在与NBU2的存在高度相关，这表明NBU2负责该基因在拟杆菌菌株中的传播。我们的结果表明，与NBU相关的元件形成了一个庞大且异质的家族，其成员具有相似的整合机制，但具有不同的靶位点，并且在是否携带抗性基因方面存在差异。
