SUMMARY: Five of eight strains of and one of 13 strains of were found to harbour DNA plasmids, pSB1 and pSB2 plasmids were isolated from strains IFO 0488 and IFO 1047, respectively, and pSB3 and pSB4 from strain IFO 1730. All four plasmids resemble 2-μm DNA of in that (i) their molecular sizes are about 6 kb, (ii) each molecule possesses a pair of inverted repeats, (iii) they exist as a mixture of two isomers and (iv) their copy numbers in the native host are similar. None of them showed homology with 2-μm DNA or with each other by Southern hybridization under moderately stringent conditions, but pSB4 hybridized with the pSR1 DNA, which was found previously in a strain of Each of the pSB plasmids has DNA sequence(s) effective for autonomous replication in In pSB3 and pSB4 showed intramolecular recombination but neither supported isomerization of 2-μm DNA.


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