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.