The resolution and regeneration of a cointegrate plasmid reveals a model for plasmid evolution mediated by conjugation and oriT site-specific recombination.

Cointegrate plasmids are useful models for the study of plasmid evolution if their evolutionary processes can be replicated under laboratory conditions. pBMB0228, a 17 706 bp native plasmid originally isolated from Bacillus thuringiensis strain YBT-1518, carries two nematicidal crystal protein genes, cry6Aa and cry55Aa. In this study, we show that pBMB0228 is in fact a cointegrate of two plasmids and contains two functional replication regions and two functional mobilization regions. Upon introduction into B. thuringiensis strain BMB171, pBMB0228 spontaneously resolves into two constituent plasmids via recombination at its oriT1 and oriT2 sites. The resolution does not require conjugation but can be promoted by conjugation. We further confirm that the resolution is mediated by oriT site-specific recombination requiring Mob02281 or Mob02282. Additionally, the two constituent plasmids of pBMB0228 are mobilizable, and can fuse back via oriT site-specific integration after entering into the same cell by conjugation. Our study confirms that native plasmid can reversibly interconvert between a cointegrate structure and its constituent plasmids. This study provides insight into the evolution of cointegrate plasmids, linking plasmid evolution with conjugation and the oriT site-specific recombination function of relaxase.