Intermolecular recombination during transformation of Bacillus subtilis competent cells by monomeric and dimeric plasmids

Bacillus subtilis competent cells harboring plasmid pUB110 were transformed by plasmids unable to replicate in this host but carrying segments of pUB110, 260 to 4500 bp long. Recombinants between the incoming and the resident plasmids were found in the transformed cells. Transforming efficiency of the incoming plasmids depended strongly on their molecular form and the length of their region homologous with the resident plasmid. It increased with the fourth to fifth power of that length for monomers having at least 900 bp of homology. Activity of monomers having less than 900 bp homology was too low to be measured in our experiments. Transforming efficiency of dimers was much greater than that of monomers, and varied with the square of the length of the homologous region. These results indicate that dimeric and monomeric plasmid molecules are processed differently during transformation of B. subtilis competent cells.