SUMMARY: 80 is a naturally occurring, penicillinase-producing strain of , which differs from the majority of such strains in that the genetic determinants for penicillinase production are located on the chromosome. The strain is also resistant to various metal ions and the genes controlling these resistances are located on a plasmid-the π plasmid. When the strain is maintained on nutrient agar slopes at room temperature, a change occurs in its genetic constitution in that these unlinked genetic elements become closely linked and co-transducible with a high frequency. The results described in this paper are consistent with the hypothesis that the change is due to a duplication of the penicillinase genes with one copy of the genes being retained in its original chromosomal location and the other copy being incorporated in the π plasmid.


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