Effect of Plasmid Incompatibility on DNA Transfer to Streptococcus cremoris

Several Streptococcus cremoris strains were used in protoplast transformation and interspecific protoplast fusion experiments with Streptococcus lactis and Bacillus subtilis, with pGKV110, pGKV21, and DeltapAMbeta1 as the marker plasmids. DeltapAMbeta1 is a 15.9-kilobase nonconjugative, deletion derivative of pAMbeta1, which is considerably larger than the pGKV plasmids (approximately 4.5 kilobases). In general, DeltapAMbeta1 was transferred more efficiently than the pGKV plasmids. Using electroporation, we were able to demonstrate that failure of efficient transfer for the pGKV plasmids was, except for one case, caused by incompatibility of these plasmids with resident plasmids of the recipient strain.