[Organization of the genes responsible for colicin Ib synthesis and immunity to it in plasmid ColIb-P9].

To study the localization and expression of the ColIb-P9 plasmid genes responsible for colicin Ib synthesis and immunity to it, we isolated a series of Tn5 insertion mutants of recombinant plasmid pIV41 containing the colicin Ib gene in EcoRI fragment of ColIb-P9 (2.7 kb) and the deletion plasmid carrying only a part of the colicin gene. The direction of colicin Ib gene transcription was determined by the analysis of the polypeptides synthesized in minicells carrying the mutant plasmids. The pIV41 plasmid containing cells have been shown to be resistant to both colicin Ib and Ia activities. This type of resistance is usually associated with chromosomal mutations resulting in loss of cell receptors for colicin Ib adsorption. Apparently, the EcoRI fragment of ColIb-P9 studied contains no gene responsible for immunity to colicin. It has been shown that this gene is a portion of SalI fragment (22 kb) of the ColIb-P9, the fragment also carrying the gene which mediates synthesis of colicin Ib.