Nucleotide sequencing of the structural gene for colicin N reveals homology between the catalytic, C-terminal domains of colicins A and N.

An 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 that cna expression is repressed by LexA protein. Deletion of the LexA binding site allowed efficient transcription of cna from an upstream lacZ promoter, whereas its presence reduced lacZ-promoted cna expression to varying extents depending on the proximity of lacZp and the SOS boxes. The molecular weight of colicin N, as deduced from the nucleotide sequence, is 41,696, which is close to the experimentally determined molecular weight of 39,000. 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 cytoplasmic membrane of Escherichia 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 of Escherichia coli cells, a more detailed investigation revealed that it too depolarizes the Escherichia coli cytoplasmic membrane and that lysis is a secondary effect.