Rice L B, Marshall S H
Medical Research Service, Department of Veterans Affairs Medical Center, Cleveland, Ohio 44106.
Antimicrob Agents Chemother. 1994 Apr;38(4):693-701. doi: 10.1128/AAC.38.4.693.
We have previously identified an inverted repeat characteristic of staphylococcal beta-lactamase transposons adjacent to the chromosomal beta-lactamase genes of Enterococcus faecalis CH19 and its beta-lactamase-producing transconjugant CX19. Nucleotide sequence analysis of the CH19 beta-lactamase structural gene (blaZ) reveals it to be identical to the blaZ gene from E. faecalis HH22 and to the blaZ gene from the staphylococcal beta-lactamase transposon Tn552. We also report the presence of nucleotide sequence identical to a 317-bp region of the staphylococcal insertion sequence IS256 upstream of the blaZ gene in both CH19 and CX19. The identical segment of IS256 is present downstream of the blaZ gene of CX19, suggesting a second insertion of the element (in the inverted orientation) accompanying transfer to the recipient strain. Restriction analysis of the areas beyond the ClaI sites used to clone these regions suggests that full copies of the IS256-like element (designated IS256E) are present in all positions but that these elements were not directly involved in the transfer of the beta-lactamase gene to the recipient strain. We have also identified a region downstream of the second IS256E insertion site which exhibits substantial homology to ISSIW, an iso-ISSI insertion originally identified in Lactococcus lactis subsp. cremoris. These data suggest that the two enterococcal blaZ genes sequenced to date evolved from a common ancestor and may at one time have been incorporated into a transposon similar to Tn552. They also suggest that IS256-like elements are mobile in E. faecalis and capable of inserting in a manner consistent with the formation of novel composite transposons. Finally, they provide the first confirmation of the presence of an ISSI-like element in enterococci, raising the possibility that these elements play a role in the exchange of chromosomal antimicrobial resistance determinants.
我们之前已确定粪肠球菌CH19及其产β-内酰胺酶的接合子CX19的染色体β-内酰胺酶基因附近存在葡萄球菌β-内酰胺酶转座子的反向重复特征。对CH19β-内酰胺酶结构基因（blaZ）的核苷酸序列分析表明，它与粪肠球菌HH22的blaZ基因以及葡萄球菌β-内酰胺酶转座子Tn552的blaZ基因相同。我们还报告了在CH19和CX19中，blaZ基因上游存在与葡萄球菌插入序列IS256的一个317 bp区域相同的核苷酸序列。IS256的相同片段存在于CX19的blaZ基因下游，这表明该元件（以反向方向）在转移至受体菌株时发生了第二次插入。对用于克隆这些区域的ClaI位点以外区域的限制性分析表明，IS256样元件（命名为IS256E）的完整拷贝存在于所有位置，但这些元件并未直接参与β-内酰胺酶基因向受体菌株的转移。我们还在第二个IS256E插入位点下游鉴定出一个区域，该区域与ISSIW具有高度同源性，ISSIW是最初在乳酸乳球菌亚种cremoris中鉴定出的一种异ISSI插入序列。这些数据表明，迄今为止测序的两个肠球菌blaZ基因起源于一个共同祖先，并且可能曾经被整合到一个类似于Tn552的转座子中。它们还表明，IS256样元件在粪肠球菌中具有移动性，并且能够以与新型复合转座子形成相一致的方式插入。最后，它们首次证实了肠球菌中存在ISSI样元件，这增加了这些元件在染色体抗菌抗性决定簇交换中发挥作用的可能性。
