Analysis of a DNA region from low-copy-number plasmid pYAN-1 ofSphingobium yanoikuyaeresponsible for plasmid stability

We identified and analyzed a DNA region that is required for the stable maintenance of plasmids in the genus Sphingomonas. This DNA fragment, a 244 bp, is localized in the upstream region of the repA gene of low-copy-number small plasmid pYAN-1 (4896 bp) of Sphingobium yanoikuyae. It has four inverted repeats and one direct repeat for possible secondary structures. We were able to stabilize not only another unstable plasmid, pYAN-2, in the genus Sphingomonas, but also the unstable plasmid pSC101 without par locus in Escherichia coli. The copy-number levels between the unstable plasmid and the parental plasmid were similar, and these results suggest that the stabilization of unstable plasmids by this DNA region of pYAN-1 was not due to an increase in plasmid copy number. We concluded that the stabilization of the plasmid was due to a plasmid partition mechanism encoded by a DNA fragment of pYAN-1.