Organization of the colicin Ib gene. Promoter structure and immunity domain.

The colicin Ib gene of the large low-copy-number plasmid Col Ib-P9 has been cloned into the plasmid vector pBR322. Observations of growth characteristics of strains carrying the cloned gene and analysis of insertion and deletion derivatives of the recombinant plasmid have led to the conclusion that colicin Ib production can occur without the colicin Ib immunity system. The direction of transcription for the colicin Ib gene was determined by use of an expression vector and subsequently confirmed by DNA-sequence analysis of the colicin Ib promoter. The physical maps of the cloned colicin Ia and colicin Ib genes exhibit a great deal of similarity which has enabled the construction of hybrid genes consisting of the N terminus of one colicin gene being linked to the C terminus of the other. From an analysis of the chimeric colicins and the locations of the fusions, we conclude that the information necessary for immunity recognition by colicin Ia and colicin Ib resides in the C-terminal half of the colicin proteins.