Transformation of Streptococcus pneumoniae with S. pneumoniae-lambda phage hybrid DNA: induction of deletions.

The genetic fate of a fragment of Streptococcus pneumoniae DNA cloned into a derivative of the Escherichia coli bacteriphage lambda has been studied in pneumococcal transformation. Transforming activity of this hybrid DNA is 8 times higher than standard S. pneumoniae DNA. Hybrid DNA is mutagenic for the recipient bacteria. Mutations are induced at a rate of 2 per 1000 transformation events. Most of these mutations are deletions adjacent to the cloned pneumococcal fragment, starting at or near its extremities and extending outside. The length of these deletions, estimated by genetic analysis or by gel electrophoresis of DNA fragments generated by restriction endonucleases, is quite variable, ranging from 150 base pairs to more than 1800 base pairs. Insertion of lambda DNA bas been detected in two large deletions by using DNAxDNA hybridization as a probe. This suggests that nonhomologous regions adjacent to the cloned fragment may be illegitimately integrated by the tranformation process. During the genetic analysis of these induced mutations we have observed that not only these deletions but also spontaneous deletions drastically increase recombination rates when present on donor DNA in transformation of neighboring markers. Such an effect is interpreted as partial exclusion of deletions from synapsis between donor and recipient DNA.