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Title: LINEAGE-SPECIFIC DECAY OF FOLATE BIOSYNTHESIS GENES SUGGESTS ON-GOING HOST ADAPTATION IN PHYTOPLASMAS

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

Submitted to: DNA and Cell Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/14/2005
Publication Date: 9/14/2005
Citation: Davis, R.E., Jomantiene, R., Zhao, Y. 2005. Lineage-specific decay of folate biosynthesis genes suggests on-going host adaptation in phytoplasmas. DNA and Cell Biology. 24:832-840.

Interpretive Summary: Many serious diseases of plants around the world are caused by very small bacteria known as phytoplasmas. Each phytoplasma cell is surrounded by a single membrane; in this way, they are like mycoplasmas - their relatives that cause diseases of animals, and spiroplasmas - relatives that include pathogens of plants. Phytoplasma cells are among the smallest cells known. In addition, phytoplasmas have some of the smallest known genomes, that is, they possess a very small number of genes. To understand how phytoplasmas survive inside their plant and insect hosts and also cause disease in spite of their small gene complement, we are comparing genes in phytoplasma with genes in other bacteria, including mycoplasma and spiroplasma. We have found that some genes needed for the synthesis of an essential vitamin were present in some phytoplasmas but have been destroyed in others. These findings will be of interest to scientists who wish to understand the evolution of phytoplasmas and the mechanisms by which phytoplasmas survive and cause disease.

Technical Abstract: Phytoplasmas are nonculturable cell wall-less, obligate intracellular pathogens of plants and insect vectors. In their descent from walled bacterial ancestors, phytoplasmas underwent massive genome reduction, resulting in some of the smallest cellular genomes known in non-symbiotic bacteria. While requirements for in vitro culture of phytoplasmas remain unknown, two opposing reports have appeared concerning genes encoding the ability of phytoplasmas to synthesize folates de novo. One study found pseudogene homologues of folP and folK, obviating folate synthesis in ‘Candidatus Phytoplasma asteris’-related strain CPh, whereas, a separate study found intact genes encoding a complete folate biosynthesis pathway in ‘Ca. Phytoplasma asteris’-related strain OY. To resolve the apparent conflict, we hypothesized that evolutionary adaptation to the availability of folate and/or other metabolites in host cells is an on-going process in the phytoplasma clade that is reflected in part by differences among phytoplasmas in the status of genes of the folate biosynthesis pathway. By studying folP and folK loci in eleven closely related phytoplasmas, we determined that these essential folate biosynthesis genes are intact in some phytoplasmas but are deteriorating in closely related strains. We suggest that the status of the folate biosynthesis pathway and the course of gene decay are lineage-specific, predicting the eventual, lineage related loss of recognizable folP and folK homologues in phytoplasma genomes.