Assembly of colicin genes from a few DNA fragments. Nucleotide sequence of colicin D

Summary The nucleotide sequence of a 2.4 kb Dral‐EcoRV fragment of pColD‐CA23 DNA was determined. The segment of DNA contained the colicin D structural gene (cda) and the colicin D immunity gene (cdi). From the nucleotide sequence it was deduced that colicin D had a molecular weight of 74683D and that the immunity protein had a molecular weight of 10057D. The amino‐terminal portion of colicin D was found to be 96% homologous with the same region of colicin B. Both colicins share the same cell‐surface receptor, FepA, and require the TonB protein for uptake. A putative TonB box pentapeptide sequence was identified in the amino terminus of the colicin D protein sequence. Since colicin D inhibits protein synthesis, it was unexpected that no homology was found between the carboxy‐terminal part of this colicin and that of the protein synthesis inhibiting colicin E3 and cloacin DF13. This could indicate that colicin D does not function in the same manner as the latter two bacteriocins. The observed homology with colicin B supports the domain structure concept of colicin organization. The structural organization of the colicin operon is discussed. The extensive amino‐terminal homology between colicins D and B, and the strong carboxy‐terminal homology between colicins B, A, and N suggest an evolutionary assembly of colicin genes from a few DNA fragments which encode the functional domains responsible for colicin activity and uptake.