Streptococcus plasmid pAMα1 is a composite of two separable replicons, one of which is closely related to Bacillus plasmid pBC16

Abstract

A tetracycline resistance plasmid of Streptococcus faecalis, pAMα1, is shown to contain two independent sets of replication functions, separated from each other on either side by short (300- to 400-base-pair) sequences of homology. The homologous sequences are oriented as direct repeats and therefore permit the dissociation of pAMα1 into its component replicons, referred to here as pAMα1Δ1 and pAMα1Δ2, as the reciprocal products of a simple intramolecular recombination. pAMα1Δ1 is a 4.6-kilobase plasmid which carries the tet gene, and pAMα1Δ2 is a 5.1-kilobase plasmid which carries no known selectable marker. pAMα1Δ1 is shown to replicate efficiently in Bacillus subtilis and to confer tetracycline resistance on Bacillus hosts. We demonstrate by restriction mapping analysis that pAMα1Δ1 is virtually identical to a 4.6-kilobase tetracycline resistance plasmid of bacillus cereus, pBC16, which is known to show extensive homology to plasmid isolates from Staphylococcus species (such as pUB110), as well as from other Bacillus species. The pAMα1Δ1-pBC16-pUB110 replicon thus exists naturally in at least three different gram-positive genera, indicating that these plasmids have a high degree of interspecific functional adaptability and supporting the view that plasmid DNA is commonly exchanged among many species of gram-positive bacteria in their natural environments.