A plasmid of phytoplasma encodes a unique replication protein having both plasmid- and virus-like domains: Clue to viral ancestry or result of virus/plasmid recombination?

The genomes of most prokaryotic and eukaryotic single-stranded (ss) DNA vir uses, and some prokaryotic plasmids such as pLS1, commonly replicate via a rolling circle replication (RCR) strategy, and thus the viruses are hypothe sized to have evolved from the plasmids, although evidence for this view is sparse. We have sequenced a circular plasmid of 3933 nt, pOYW, obtained fr om onion yellows phytoplasma (OY-W), a cell-wall-less, unculturable prokary ote that inhabits the cytoplasm of both plant and insect cells. pOYW contai ns five open reading frames (ORFs) on the same strand and apparently replic ates by an RCR mechanism. Its rep gene (ORF5) encodes a unique protein, pOY W-Rep, with an unprecedented structure. The N-terminal region of pOYW-Rep h as similarities to the RCR initiator protein (Rep) of pLS1 family plasmids but, unlike the Rep of other plasmids, its C;terminal region was unexpected ly similar to the helicase domain of the replication-associated proteins (R ap) of eukaryotic viruses, especially circoviruses (ssDNA viruses of verteb rates). The pOYW-Rep was specifically detected in OY-W-infected plant phloe m cells, suggesting that it is a functional protein. We suggest that an anc estral phytoplasma plasmid pOYW may have acquired a helicase domain from ho st phytoplasmal DNA, entered the surrounding eukaryotic cytoplasm, and subs equently evolved into an ancestral eukaryotic ssDNA virus. Alternatively, a pOYW ancestor could have obtained the helicase domain by recombination wit h a virus: this would be the first example of recombination between plasmid s and viruses. (C) 2001 Academic Press.