Transformation of Streptococcus sanguis (Challis) by linear plasmid molecules.

The streptococcal erythromycin resistance plasmid pSM9 was used to study the problem of how the transforming activity of mixtures of two unique linear products of restriction enzyme digestion depends on the distance between the cleavage sites. In transformation of the Challis strain of S. sanguis, the transforming activity of mixed digests increased with increasing relative distances (x) between the restriction sites, where 0 less than or equal to X less than or equal to 0.5. To explain the experimental results, a mathematical model was proposed according to which the overall probability (p) of transformation resulting in a functional replicon is the product of the partial probabilities of initial single-strand pairing, circularization, and stability of the paired intermediate, all of which were assumed to depend on x. A linear relationship found between transformation frequency and p was taken to support the model. Transformation of Challis by mixtures of two linearized plasmid molecules with regions of internal nonhomology resulting in paired intermediates with insertion or substitution loops allowed either donor molecule to contribute to the transformation yield.