DNA sequence analysis of three missense mutations affecting colicin E3 bactericidal activity

We have determined the nucleotide sequence changes caused by three missense mutations leading to the production of inactive colicin E3 proteins. The ceaC1 mutation, affecting the translocation of colicin E3 through bacterial membranes, is caused by a serine to phenylalanine change at position 37 within the glycine-rich region at the N-terminal part of colicin E3. This confirms previous results suggesting a role for this domain in colicin uptake by sensitive cells. The ceaC2 and ceaC3 mutations, abolishing colicin E3 RNase activity, affect the C-terminal enzymatic domain of the molecule. In the ceaC2 mutant, serine at position 529 was converted to leucine. The ceaC3 mutation replaced a glycine residue at position 524 with an aspartic acid residue. The two mutations ceaC2 and ceaC3 yield information on the amino acid residues involved in the RNase activity of colicin E3.