Site-directed mutagenesis was used to replace the codon for the N-terminal cysteine residue of pColE2-P9-encoded mature lysis protein (CelB) by an arginine codon. In contrast to the wild-type CelB protein, the product of the mutated gene, which has an altered signal peptidase cleavage site, was neither processed nor acylated. However, the mutant protein retained sufficient residual activity to cause partial, Mg2+-suppressible lysis and could activate envelope phospholipase A1-A2 and promote colicin release, albeit with reduced efficiency compared to the wild-type protein. We propose that the uncleaved signal peptide of the mutant protein acts as the functional equivalent of the fatty acyl groups normally linked to the N-terminal cysteine residue of the wild-type protein, thereby anchoring the protein in the cell envelope where it exerts its various effects.