Genetic analysis of the bacteriocin-encoding plasmids pRJ6 and pRJ9 of Staphylococcus aureus by transposon mutagenesis and cloning of genes involved in bacteriocin production

pRJ6 and pRJ9, small Staphylococcus aureus plasmids which code for bacterio cins, exhibited a bactericidal activity against several lactic acid bacteri a and strains of Listeria monocytogenes, an important food-borne pathogen. Filter-mating experiments using plasmid derivatives tagged with either Tn55 1 or Tn917-lac showed that pRJ6, but not pRJ9, could be mobilized by staphy lococcal conjugative plasmids. Transposon mutagenesis of both plasmids was also performed. The bacteriocin and immunity structural genes of pRJ6 are p art of the same operon, which is located around co-ordinate 4.0, beings tra nscribed from right to left. However, gene cloning experiments using a stap hylococcal vector showed some evidence for the involvement of additional fu nctions of pRJ6 in bacteriocin expression. One function involved in pRJ6 mo bilization mapped around co-ordinate 5.2, and it appears to be transcribed from left to right. The bactericidal action exerted by strains harbouring p RJ9 appears to reflect the activity of at least two bacteriocins, whose com bined action results in a broader spectrum of activity and in a higher anta gonistic activity. Gene cloning experiments also supported these assumption s.