A series of plasmids encoding hybrid proteins comprising various lengths of the NH2-terminal region of colicin N coupled to almost complete beta-galactosidase or alkaline phosphatase polypeptides was constructed by transposon mutagenesis of ColN plasmid derivatives. Synthesis of the hybrid proteins, like that of colicin N itself, was regulated by the SOS response. Large quantities of the hybrid proteins accumulated in the cytoplasm (beta-galactosidase) or particulate fractions (alkaline phosphatase). When the gene fusions were expressed in cells that were producing colicin E2 and expressing the ColE2 lysis gene, only very low levels of the hybrid proteins were found in the medium. The results suggest that the amino-terminal part of colicin N does not contain sufficient biochemical information to promote the release of the hybrid proteins into the medium.