Projan S J, Monod M, Narayanan C S, Dubnau D
Department of Plasmid Biology, Public Health Research Institute, New York, New York 10016.
J Bacteriol. 1987 Nov;169(11):5131-9. doi: 10.1128/jb.169.11.5131-5139.1987.
A naturally occurring plasmid from Bacillus subtilis, pIM13, codes for constitutively expressed macrolide-lincosamide-streptogramin B (MLS) resistance, is stably maintained at a high copy number, and exists as a series of covalent multimers. The complete sequence of pIM13 has been reported (M. Monod, C. Denoya, and D. Dubnau, J. Bacteriol. 167:138-147, 1986) and two long open reading frames have been identified, one of which (ermC') is greater than 90% homologous to the ermC MLS resistance determinant of the Staphylococcus aureus plasmid pE194. The second reading frame (repL) shares homology with the only long open reading frame of the cryptic S. aureus plasmid pSN2 and is probably involved in plasmid replication. The map of pIM13 is almost a precise match with that of pE5, a naturally occurring, stable, low-copy-number, inducible MLS resistance plasmid found in S. aureus. pIM13 is unstable in S. aureus but still multimerizes in that host, while pE5 is unstable in B. subtilis and does not form multimers in either host. The complete sequence of pE5 is presented, and comparison between pIM13 and pE5 revealed two stretches of sequence present in pE5 that were missing from pIM13. It is likely that a 107-base-pair segment in the ermC' leader region missing from pIM13 accounts for the constitutive nature of the pIM13 MLS resistance and that the lack of an additional 120-base-pair segment in pIM13 that is present on pE5 gives rise to the high copy number, stability, and multimerization in B. subtilis. The missing 120 base pairs occur at the carboxyl-terminal end of the putative replication protein coding sequence and results in truncation of that protein. It is suggested either that the missing segment contains a site involved in resolution of multimers into monomers or that the smaller replication protein causes defective termination of replication. It is concluded that pIM13 and pE5 are coancestral plasmids and it is probable that pIM13 arose from pE5.
来自枯草芽孢杆菌的天然质粒pIM13编码组成型表达的大环内酯-林可酰胺-链阳菌素B（MLS）抗性，能以高拷贝数稳定维持，并以一系列共价多聚体形式存在。pIM13的完整序列已被报道（M. 莫诺德、C. 德诺亚和D. 杜布瑙，《细菌学杂志》167:138 - 147，1986年），并已鉴定出两个长开放阅读框，其中一个（ermC'）与金黄色葡萄球菌质粒pE194的ermC MLS抗性决定簇有90%以上的同源性。第二个阅读框（repL）与隐性金黄色葡萄球菌质粒pSN2的唯一长开放阅读框有同源性，可能参与质粒复制。pIM13的图谱与pE5几乎完全匹配，pE5是在金黄色葡萄球菌中发现的天然存在、稳定、低拷贝数、可诱导的MLS抗性质粒。pIM13在金黄色葡萄球菌中不稳定，但在该宿主中仍会多聚化，而pE5在枯草芽孢杆菌中不稳定，在两种宿主中都不形成多聚体。本文给出了pE5的完整序列，pIM13与pE5的比较揭示了pE5中存在而pIM13中缺失的两段序列。pIM13的ermC'前导区中缺失的一个107个碱基对的片段可能是pIM13 MLS抗性组成型性质的原因，而pIM13中缺少pE5上存在的另外一个由120个碱基对组成的片段导致了其在枯草芽孢杆菌中的高拷贝数、稳定性和多聚化。缺失的120个碱基对出现在假定的复制蛋白编码序列的羧基末端，导致该蛋白截短。有人认为，缺失的片段要么包含一个参与将多聚体分解为单体的位点，要么较小的复制蛋白导致复制的终止有缺陷。得出的结论是，pIM13和pE5是共同祖先质粒，pIM13很可能起源于pE5。
