Summary: In several strains of extensive breakdown of chromosomal DNA may be potentiated by osmotic lysis of protoplasts. At its most severe, in strains originating from Farmer & Rothman's thymine auxotroph, the rate of DNA breakdown was greater than 50% per hour at 40 °C. The rate of DNA breakdown in most other strains tested was approximately 5% per hour except for SP.-strains, in which the rate of DNA breakdown was only 0.3%. DNA degradation was attributed to relaxation of control of a nuclease specified by the prophage of SP. or a related phage. The most potent nuclease in lysates was an ATP-activated protein of 280000. Derivatives of Farmer and Rothman's strain containing integrated plasmids had the highest rate of DNA degradation. Although the chromosome was completely destroyed, covalently closed circular plasmids were generated from the integrated sequence. These showed massive deletions of the part of the integrated plasmid but the vector sequence remained intact. The nucleolytic activity therefore appears to recognize specific sequences in DNA. We suggest that activation of SP. genes during development of competence may be a cause of deletion of cloned genes in the early stages of establishment of cloned sequences.


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