Summary: Stable ampicillin-resistant transformants of PCC 6301 were isolated using as donor fragments chromosomal DNA cloned into ColEl-derived vectors. Using dark-incubated recipient cells, transformation was achieved reproducibly at frequencies of approximately 10- per cell and 3 × 10 per μg of donor DNA. These frequencies were 10- to 10-fold lower than those reported previously by other workers for the closely related strain PCC 7942 but only 5- to 10-fold lower than found by us in parallel experiments with both strains. Different donor fragments of PCC 6301 DNA gave different characteristic transformation frequencies which were greatly reduced by short internal deletions. The results are most simply explained by a model of integration of circular plasmid DNA into the recipient chromosome by a single crossover event.


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