摘要
      传统上，插入序列（IS）通过转位和易位在细菌中传播抗微生物耐药性基因（ARGs）方面发挥着重要作用，形成含有多个ARGs的区域，这些区域两侧是单拷贝或多拷贝的IS，据报道，缺乏重组酶基因但被不含转座酶基因的各种大小的直接重复序列（DR）包围的ARGs参与了ARGs的传播。在本研究中，在E.faecalis E508的质粒pE508–2上鉴定了一种新的UCS，该质粒携带24411bp的多抗性基因簇，由抗性基因aphA3、lnu（B）、lsa（E）、spw、aac（a）-aph（D）、lnu（B）和dfrG组成，两个拷贝的aadE两侧有erm（B）。PCR分析显示，形成了三种长度分别为7235、16437和23673bp的UCS，每种都含有各自的抗性基因和一个erm（B）拷贝。使用erm（B）阴性和阳性菌株，我们证明了携带erm（B）的UCSs不能转移到erm（乙）阴性菌株中，但可以通过自然转化在与先前存在的erm（乙）基因相邻的新位点整合到一个erm（丙）阳性菌株中。数据库搜索显示，erm（B）侧翼的多抗性基因区域可能能够形成各自的UCS，存在于不同国家不同来源的各种细菌中。总之，本研究通过实验证明了UCS的切除和整合涉及包括erm（B）的结构。这些UCS在各种革兰氏阳性细菌中的广泛存在突出了其在细菌病原体中ARGs传播中的作用。
Abstract
Traditionally, insertion sequences (ISs) play a major role in disseminating antimicrobial resistance genes (ARGs) in bacteria through transposition and translocation, forming regions that contain multiple ARGs flanked by single or multiple copies of IS. In addition, unconventional circularizable structures (UCSs), lacking recombinase genes but being surrounded by directly repeated sequences (DRs) of various sizes which do not contain transposase genes, were reported to be involved in the dissemination of ARGs. In this study, a novel UCS was identified on plasmid pE508–2 in E. faecalis E508, which carried a 24,411 bp multiresistance gene cluster, consisting of the resistance genes aphA3, lnu(B), lsa(E), spw, aac(A)-aph(D), lnu(B), dfrG, and two copies of aadE flanked by copies of erm(B). PCR assays revealed that three types of UCSs with lengths of 7235, 16,437, and 23,673 bp were formed, each of which contained the respective resistance genes and one copy of erm(B). Using erm(B)-negative and -positive strains, we demonstrated that erm(B)-carrying UCSs failed to transfer into an erm(B)-negative strain, but could integrate into an erm(B)-positive strain in a new site adjacent to a pre-existing erm(B) gene by natural transformation. Database searches revealed that erm(B)-flanked multiresistance gene regions, which might be able to form the respective UCSs, are present among various bacteria from different sources in various countries. In summary, this study experimentally demonstrated the excision and integration of UCS involving structures that include erm(B). The widespread presence of these UCSs in various Gram-positive bacteria highlights its role in the dissemination of ARGs among bacterial pathogens.

https://www.sciencedirect.com/science/article/abs/pii/S0378113522002127