The identification and conservation of tunicaminyluracil-related biosynthetic gene clusters in several Rathayibacter species collected from Australia, Africa, Eurasia and North America

Authors: Tancos, Matthew; Sechler, Aaron; DAVIS II, EDWARD - Oregon State University; CHANG, JEFF - Oregon State University; SCHROEDER, BRENDA - University Of Idaho; MURRAY, TIMOTHY - Washington State University; Rogers, Elizabeth

Submitted to: Frontiers in Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/3/2019
Publication Date: 1/10/2020
Citation: Tancos, M.A., Sechler, A.J., Davis, E.W., Chang, J.H., Schroeder, B.K., Murray, T.D., Rogers, E.E. 2020. The identification and conservation of tunicaminyluracil-related biosynthetic gene clusters in several Rathayibacter species collected from Australia, Africa, Eurasia and North America. Frontiers in Microbiology. 10:2914. https://doi.org/10.3389/fmicb.2019.02914.
DOI: https://doi.org/10.3389/fmicb.2019.02914

Interpretive Summary: Rathayibacter toxicus is a plant pathogenic bacterium that is a potential threat to U.S. agriculture and food security. R. toxicus is transmitted to a variety of forage grasses by plant parasitic nematodes. Once the bacterium colonizes the seed head of the plant it is able to produce a toxin that can kill grazing livestock when consumed. To date, R. toxicus has only been identified in Australia, but due to its potential impact on U.S. agriculture it was listed as a U.S. select agent. It was previously thought that only R. toxicus was capable of producing this specific toxin. However, we sequenced the genome of numerous Rathayibacter strains and identified similar toxin gene clusters in three additional Rathayibacter species isolated from the U.S. Pacific Northwest, South Africa, and the Middle East. The genetic data suggests that the toxin gene clusters have been independently acquired in these three Rathayibacter species. Knowing the global distribution of all toxin producing Rathayibacter species is important for management, and will allow us to better understand the shared triggers of toxin production.

Technical Abstract: Tunicaminyluracil antibiotics are a novel class of toxigenic glycolipids that are synthesized by several soil-associated Actinomycetes. The acquisition of a tunicaminyluracil biosynthetic gene cluster (TGC) in Rathayibacter toxicus has led to the emergence of the only described, naturally occurring tunicaminyluracil-associated mammalian disease, annual ryegrass toxicity of livestock. Endemic to Australia, R. toxicus is obligately vectored by Anguinid seed gall nematodes to the developing seedheads of forage grasses, in which the bacteria synthesize tunicaminyluracils that may subsequently be consumed by livestock and result in high rates of mortality and morbidity. The potential impact of R. toxicus on U.S. agriculture has led the U.S. Department of Agriculture – Animal and Plant Health Inspection Service to list R. toxicus as a Plant Pathogen Select Agent. R. toxicus is the only characterized phytopathogenic bacterium to produce tunicaminyluracils, but numerous R. toxicuslike livestock poisonings outside Australia suggest additional bacterial sources of tunicaminyluracils may exist. To investigate the conservation of the TGC in R. toxicus and whether the TGC is present in other Rathayibacter species, we analyzed genome sequences of members of the Rathayibacter genus. Putative TGCs were identified in genome sequences of R. toxicus, R. iranicus, R. agropyri, and an undescribed South African Rathayibacter species. In the latter three species, the putative TGCs have homologs of tunicaminyluracil-related genes essential for toxin production, but the TGCs differ in gene number and order. The TGCs appear at least partially functional because in contrast to atoxigenic species, TGC-containing Rathayibacter species were each able to tolerate exogenous applications of tunicamycin from Streptomyces chartreusis. The North American R. agropyri TGC shows extensive diversity among the sequenced isolates, with presense/absense polymorphisms in multiple genes or even the whole TGC. R. agropyri TGC structure does not appear to correlate with date or location of isolate collection. The conservation and identification of tunicaminyluracil-related gene clusters in three additional Rathayibacter species isolated from South Africa, the Middle East, and the United States, suggests a wider global distribution of potentially neurotoxigenic plant-associated bacteria. This potential for additional endemic and exotic toxigenic Rathayibacter species could have widespread and severe implications for agriculture.