PROTECTION AGAINST BACTERIOCIN 28B IN SERRATIA-MARCESCENS IS APPARENTLY NOT RELATED TO THE EXPRESSION OF AN IMMUNITY GENE

The gene encoding bacteriocin 28b from Serratia marcescens N28b (bss g ene) has been cloned in Escherichia coli and its nucleotide sequence h as been determined. The genetic determinants coding for other well-cha racterized bacteriocins from enterobacteria (colicins) are located in plasmids and they have always been shown to contain a gene responsible for immunity located downstream from the bacteriocin structural gene. In some cases there is another gene located downstream from the immun ity gene, which is responsible for bacteriocin release. Analysis of ba cteriocin 28b release and the sensitivity to this bacteriocin of E. co li strains harbouring recombinant plasmids containing the bss gene sho wed that bacteriocin 28b is not released from the cell in these strain s and that their phenotypic insensitivity is not associated with any r egion close to the structural gene. The nucleotide sequence of the reg ion downstream from the bss gene contains two putative open reading fr ames transcribed in the opposite direction to the bss gene. These open reading frames apparently encode proteins that seem not to be involve d in bacteriocin immunity or release. Moreover, a S. marcescens N28b g enomic library was screened and no immunity gene was found. Therefore, bacteriocin 28b differs greatly from the bacteriocins from other ente robacteria, and in the following senses it is unique: firstly, the gen e encoding bacteriocin 28b seems to be located on the chromosome, and secondly, insensitivity to this bacteriocin in S. marcescens N28b is n ot associated with the expression of an immunity gene.