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Research Paper |
Graduate School of Agricultural and Life Sciences, The University of Tokyo, Kashiwa, Chiba 277-8562, Japan1
Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa, Chiba 277-8562, Japan2
Author for correspondence: Shigetou Namba. Tel: +81 4 7136 3700. Fax: +81 4 7136 3701. e-mail: snamba{at}k.u-tokyo.ac.jp
Recombination among bacterial extrachromosomal DNAs (EC-DNAs) plays a major evolutionary role by creating genetic diversity, and provides the potential for rapid adaptation to new environmental conditions. Previously, a 7 kbp EC-DNA, EcOYW1, with a geminivirus-like rolling-circle-replication protein (Rep) gene was isolated and characterized from an original wild-type line (OY-W) of onion yellows (OY) phytoplasma, an endocellular cell-wall-less prokaryote that inhabits the cytoplasm of both plant and insect cells. EcOYW1, found in OY-W, was not present in a mild-symptom line (OY-M) derived from OY-W. A 4 kbp EC-DNA, pOYW, was also isolated and characterized from OY-W, and its pLS1-plasmid-like rep gene was expressed. This paper describes the isolation and sequencing of an EC-DNA of 5560 nt, EcOYW2, from OY-W, and its counterpart EC-DNA of 5025 nt, EcOYM, from OY-M. EcOYW2 and EcOYM contained seven and six ORFs, respectively. They both encoded a geminivirus-like Rep and a putative single-stranded-DNA-binding protein (SSB). Southern blot analysis indicated that no more EC-DNAs with a rep gene exist in either OY-W or OY-M, which means that the complete set of EC-DNAs has been cloned from the OY-W and OY-M lines of OY phytoplasmas. Sequence analysis revealed that both EcOYW2 and EcOYM have chimeric structures of previously characterized EcOYW1 and pOYW, suggesting that they have a recombinational origin. This is the first evidence of intermolecular recombination between EC-DNAs in phytoplasma. The possible implications of these findings in increasing the biological diversity of phytoplasma are discussed.
Keywords: phytoplasma, extrachromosomal DNA, rolling circle replication, recombination
Abbreviations: EC-DNA, extrachromosomal DNA; OY, onion yellows; OY-M, mild-symptom line of onion yellows phytoplasma; OY-W, wild-type onion yellows phytoplasma
b The GenBank accession numbers for EcOYW and EcOYM are AB076262 and AB076263, respectively.
a Present address: Graduate School of Agriculture, Ibaraki University, Ami, Inashiki-gun, Ibaraki 300-0393, Japan.
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