Nucleotide sequences from the colicin E8 operon: homology with plasmid ColE2-P9

The primary structures of the immunity (Imm) and lysis (Lys) proteins, and the C-terminal 205 amino acid residues of colicin E8 were deduced from nucleotide sequencing of the 1,265 bp ClaI-PvuI DNA fragment of plasmid ColE8-J. The gene order is col-imm-lys confirming previous genetic data. A comparison of the colicin E8 peptide sequence with the available colicin E2-P9 sequence shows an identical receptor-binding domain but 20 amino acid replacements and a clustering of synonymous codon usage in the nuclease-active region. Sequence homology of the two colicins indicates that they are descended from a common ancestral gene and that colicin E8, like colicin E2, may also function as a DNA endonuclease. The native ColE8 imm (resident copy) is 258 bp long and is predicted to encode an acidic protein of 9,604 mol. wt. The six amino acid replacements between the resident imm and the previously reported non-resident copy of the ColE8 imm ([E8 imm]) found in the ribonuclease-producing ColE3-CA38 plasmid offer an explanation for the incomplete protection conferred by [E8 Imm] to exogenously added colicin E8. Except for one nucleotide and amino acid change in the putative signal peptide sequence, the ColE8 lys structure is identical to that present in ColE2-P9 and ColE3-CA38.