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Title: Sequence variable mosaics: Composites of recurrent transposition characterizing the genomes of phylogenetically diverse phytoplasmas

Author
item JOMANTIENE, R - VILNIUS LITHUANIA
item Zhao, Yan
item Davis, Robert

Submitted to: DNA and Cell Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/10/2007
Publication Date: 9/1/2007
Citation: Jomantiene, R., Zhao, Y., Davis, R.E., Zhao, Y. 2007. Sequence variable mosaics: Composites of recurrent transposition characterizing the genomes of phylogenetically diverse phytoplasmas. DNA and Cell Biology. 26:557-564.

Interpretive Summary: Phytoplasmas are very small bacteria that are responsible for hundreds of diseases affecting agriculturally important plants around the world. Phytoplasmas are unlike most bacteria, in that they have no cell wall, and they cannot be isolated in laboratory culture. There is need to curb the spread of phytoplasmas and to devise safe measures to control the diseases they cause, but how they cause plant diseases is not understood. In a previous study of the genome (all of the genes) in a phytoplasma, we discovered that the genome organization includes a previously unrecognized mosaic of diverse genes that occurs in multiple copies and could account for the rapid evolution of phytoplasmas, as well as the substantial differences among closely related phytoplasmas in the sizes of their genomes. These mosaics, clusters of varied genes, may be related to the ability of phytoplasmas to adapt to their environments and to cause disease. In the present communication we describe the discovery of these gene clusters in different phytoplsma species, leading to the concept that the origin of clusters is an ancient event in phytoplasma evolution and supporting the hypothesis that genes in the clusters play a role in causing plant diseases. In addition, we present new concepts on how these clusters may have formed. This work will interest scientists and students studying the evolution of bacteria, plant diseases, and molecular bases of disease mechanisms, as well as to diagnostics companies and centers involved in pathogen detection.

Technical Abstract: Phytoplasmas are cell wall-less prokaryotes characterized by small, AT-rich genomes that encode capabilities for obligate, transkingdom parasitism and pathogenicity in plants and insect vectors. Inability to isolate and characterize phytoplasmas in pure culture has led to adoption of the ‘Candidatus species’ convention to refer to distinct phytoplasma lineages. In this study, we provide evidence that multiple, sequence-variable mosaics (SVMs) of clustered genes and repetitive extragenic palindromes are characteristic features of phytoplasma genome architecture in phylogenetically diverse species. The findings suggest that SVM formation was an ancient event in evolution of the phytoplasma clade. Sequence polymorphisms of hyper-variable regions indicated rapid evolution possibly involving capture of mobile elements recurrently targeted to SVMs. Multiple events of targeted mobile element attack, recombination, and rearrangement conceivably account for the composite structure of SVMs. Proteins encoded by the highly variable region included a lysophospholipase and other putatively secreted and/or transmembrane, cell surface-interacting proteins potentially significant in phytoplasma-host interactions.