The four plasmids in E. coli 39R861 have been used as size standards for closed-circular plasmid DNA since the 1980s. However, their sizes had only been estimated. Here, the complete sequence of the 61.3 kb cryptic FII plasmid p39R861-3 is reported, and the sizes of p39R861-4, pSa and NTP168 were determined to be 155.8, 40.1 and 6.8 kb, respectively. An improved size standard, 39R861+, was created by adding two sequenced, compatible plasmids to 39R861 via conjugation. The 94.8 kb conjugative B/O plasmid p838B-R mobilised the cryptic, 2.0 kb pBuzz. The six plasmids in 39R861+ are all stably maintained.

中文翻译：

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改良的质粒大小标准品39R861。

自1980年代以来，大肠杆菌39R861中的四个质粒已用作封闭环状质粒DNA的大小标准。但是，它们的大小仅是估计值。在此，报告了61.3 kb的FII隐性质粒p39R861-3的完整序列，并且确定p39R861-4，pSa和NTP168的大小分别为155.8、40.1和6.8 kb。通过结合将两个测序的，相容的质粒添加到39R861中，从而创建了大小改进的标准品39R861 +。94.8 kb的结合型B / O质粒p838B-R动员了隐秘的2.0 kb pBuzz。39R861 +中的六个质粒均被稳定地维持。