Location of the endA gene coding for endonuclease I on the physical map of the Escherichia coli K-12 chromosome

The endonuclease I of Escherichia coli is an RNA-inhibit-able DNase which is located in the periplasmic space of the E. coli cell (3); it cleaves native DNA into oligonucleotides (7). It is assumed that in the noninhibited, RNA-free state the enzyme introduces nucleotide sequence-independent double-strand breaks into DNA (2, 9), whereas the inhibited, RNA-complexed enzyme probably causes only nicking ofDNA in the presence of high concentrations of salt (5). The enzyme is not essential for the cell, and its physiological role is unknown. Endonuclease I-deficient mutants resemble the wild type in growth rate, ability to propagate phage, and conjugational properties (4). The endA mutations were mapped near 64 min on the genetic map of the E. coli chromosome (1). We report the location of this gene on the physical map of E. coli and the direction of its transcription.