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Journal of Bacteriology, May 2006, p. 3682-3696, Vol. 188, No. 10
0021-9193/06/$08.00+0 doi:10.1128/JB.188.10.3682-3696.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Adam Ewing,1,
Sally A. Miller,2
Agnes Jancso Radek,3,
Dmitriy V. Shevchenko,3
Kiryl Tsukerman,3
Theresa Walunas,3
Alla Lapidus,3,¶
John W. Campbell,3,|| and
Saskia A. Hogenhout1*
Department of Entomology,1 Department of Plant Pathology, The Ohio State University, Ohio Agricultural Research and Development Center, Wooster, Ohio 44691,2 Integrated Genomics, Chicago, Illinois 606123
Received 30 September 2005/ Accepted 23 January 2006
Phytoplasmas
("Candidatus Phytoplasma," class
Mollicutes) cause disease in hundreds of economically
important plants and are obligately transmitted by sap-feeding insects
of the order Hemiptera, mainly leafhoppers and psyllids. The 706,569-bp
chromosome and four plasmids of aster yellows phytoplasma strain
witches' broom (AY-WB) were sequenced and compared to the onion yellows
phytoplasma strain M (OY-M) genome. The phytoplasmas have small
repeat-rich genomes. This comparative analysis revealed that the
repeated DNAs are organized into large clusters of potential mobile
units (PMUs), which contain tra5 insertion
sequences (ISs) and genes for specialized sigma factors and membrane
proteins. So far, these PMUs appear to be unique to phytoplasmas.
Compared to mycoplasmas, phytoplasmas lack several recombination and
DNA modification functions, and therefore, phytoplasmas may use
different mechanisms of recombination, likely involving PMUs, for the
creation of variability, allowing phytoplasmas to adjust to the diverse
environments of plants and insects. The irregular GC skews and the
presence of ISs and large repeated sequences in the AY-WB and OY-M
genomes are indicative of high genomic plasticity. Nevertheless,
segments of
250 kb located between the lplA and
glnQ genes are syntenic between the two phytoplasmas and
contain the majority of the metabolic genes and no ISs. AY-WB appears
to be further along in the reductive evolution process than OY-M. The
AY-WB genome is
154 kb smaller than the OY-M genome, primarily
as a result of fewer multicopy sequences, including PMUs. Furthermore,
AY-WB lacks genes that are truncated and are part of incomplete
pathways in OY-M.

Authors' contributions: X.B., performance of the majority of the bioinformatics analysis (annotation, comparative genome analyses, defining metabolic pathways
of AY-WB, and submission of sequences to GenBank), and writing of manuscript; J.Z., development of DNA isolation method, DNA isolation, gap closure, and annotation of selected sequences; A.E., annotation of selected sequences and characterization of PMUs; S.A.M., project initiation, project support, and providing materials and resources; A.J.R., sequencing and gap closure; D.V.S., construction of AY-WB genomic libraries, sequencing, and gap closure; K.T., bioinformatics (sequence assembly and gap closure); T.W., bioinformatics (maintenance of annotation database and automated annotation); A.L., project manager for construction of AY-WB genomic libraries, sequencing, and gap closure; J.W.C., project manager for bioinformatics (sequence assemply, gap closure, maintenance of annotation database, and assembly); S.A.H., project initiation, overall project management (experimental work, annotation, and all other bioinformatics analyses), and writing of manuscript.
Present address: Potato Research Center, Agriculture and Agri-Food Canada, Fredericton, NB E3B 4Z7, Canada.
Present address: GCB Graduate Group, University of Pennsylvania, Philadelphia, PA 19104.
Present address: Epicentre Technologies Corp., Madison, WI
53713.
¶ Present address: Microbial Genomics, DOE Joint Genome Institute, Walnut Creek, CA 94598.
|| Present address: Scarab Genomics, LLC, Madison, WI 53713.
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