Sulfolobus islandicus plasmids pRN1 and pRN2 share distant but common evolutionary ancestry

The complete sequence of the plasmid pRN2 from the thermoacidophile Sulfolobus islandicus has been determined. The plasmid was found to be circular and 6959 bp in length. S. islandicus harbors another endogenous plasmid, pRN1, and comparison of pRN1 and pRN2 revealed that these two plasmids are essentially homologous, although very distantly related. pRN1 and pRN2 share several stretches of highly conserved noncoding DNA and three common open reading frames. Two of these reading frames are likely related to replication, one encoding a large protein with a helicase domain similar to viral helicases, and the other a copy number control protein, CopG.