SUMMARY: The staphylococcin-like peptide Pep 5 is produced by the penicillin resistant strain 5. This strain is immune to the peptide. Plasmid analysis of 5 by agarose gel electrophoresis and electron microscopy demonstrated five plasmids with molecular weights ranging from 5·8 × 10 to 29 × 10. Variants of 5 not producing Pep 5 or which had become penicillin sensitive were induced by various curing treatments. Strains lacking the 13·9 × 10 mol. wt plasmid (pED502) had lost penicillin resistance, and those lacking the 12.3 × 10 mol. wt plasmid (pED503) failed to produce Pep 5. pED503 is also responsible for the immunity of the producer cell to Pep 5. Plasmid pED502 could be transformed into RN 981 which then became resistant to penicillin. pED503 could not be transformed into RN 981, but could be transformed into 5 variants previously cured of this plasmid; the transformants then regained the properties of Pep 5 production and immunity.


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