SummaryAn 1800 bp fragment of DNA from a natural ColN plasmid (pCHAP4) encompassing the colicin N structural gene (cna) and its regulatory region was subjected to nucleotide sequencing and deletion analysis. The region of DNA immediately upstream from cna contains two tandemly‐arranged and overlapping potential LexA binding sites (SOS boxes), in line with the previous demonstration thatcnaexpression is repressed by LexA protein. Deletion of the LexA binding site allowed efficient transcription ofcnafrom an upstreamlacZpromoter, whereas its presence reduced lacZ‐promotedcnaexpression to varying extents depending on the proximity oflacZpand the SOS boxes. The molecular weight of colicin N, as deduced from the nucleotide sequence, is 41696, which is ciose to the experimentally determined molecular weight of 39000. Colicin N has a glycine‐rich amino terminus similar to that found in many other colicins. Part of the glycine‐rich domain of colicin N could be replaced by an unrelated sequence devoid of glycine residues without affecting either colicin release or activity. The carboxy‐terminal half of colicin N exhibits significant homology to the C‐terminus of colicin A. The latter colicin forms pores In the cyto‐plasmic membrane ofEscherichia coli, thereby depolarizing the membrane and causing cell death. The C‐terminus of colicin A is endowed with this catalytic activity. Although colicin N was previously found to cause lysis ofEscherichia colicells, a more detailed Investigation revealed that it too depolarizes theEscherichia colicytopiasmic membrane and that lysis is a s