Endonuclease A degrades chromosomal and plasmid DNA of Escherichia coli present in most preparations of single stranded DNA from phagemids.

With E. coli, large and variable amounts of chromosomal and plasmid DNAs are observed in the supernatants of overnight cultures when the cells carry an endA mutation, but are not detected by gel electrophoresis when the cells carry the wild type allele of endA. Significant amounts of nuclease activity in DH11S endA+ supernatants were detected by two simple assays; the rapid degradation of added pBR322 plasmid DNA, as judged by agarose gel electrophoresis, and a decrease of more than 100000 fold in transformation efficiency of the added pBR322 plasmid DNA. By employing isogenic endA mutant and wild type strains of DH11S and DH10B/F' proAB+ laclq Z delta M15, it was shown that detectable levels of chromosomal and plasmid DNAs are observed only in the endA mutant strains. These results indicate that Endonuclease I activity is responsible for degradation of chromosomal and plasmid DNA usually present in preparations of ssDNA. Therefore, a wild type endA gene is useful for the rapid and simple production of highly purified ssDNA from cells containing phagemid vectors.