An improved plasmid size standard, 39R861

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. •The four plasmids in E. coli 39R861 have been used as plasmid size standards since the 1980s.•The sizes of plasmids in 39R861, previously only estimated, were accurately determined here.•The complete sequence of the 61.3 kb cryptic FII plasmid 39R861–3 is reported for the first time.•Two sequenced plasmids (94.8 and 2.0 kb) were added to 39R861 to generate an improved plasmid size standard, 39R861 + .•39R861+ contains six plasmids with sizes 155.8, 94.8, 61.3, 40.1, 6.8 and 2.0 kilobases.