Professor Clive Ronson

Research Interest: Molecular and Environmental Microbiology
Year of Appointment: 1991
Training: BSc(Massey) PhD(Warw)

Professor Ronson's research interests are in the areas of the plant-microbe interactions involved in symbiotic nitrogen fixation, and the role of horizontal gene transfer in microbial evolution. The major research area in the lab focuses on genomic analysis of the bacterium Mesorhizobium loti that is the microsymbiont of the model legume Lotus japonicus. In this research, we collaborate with several overseas groups characterizing the plant’s contribution to the symbiosis using L. japonicus. An important discovery we made was that new field strains of Lotus-nodulating rhizobia evolve in a single step by lateral transfer of a chromosomally-located symbiotic element from an inoculum strain to indigenous non-symbiotic rhizobia. The 502-kb element integrates into a tRNA gene in a process mediated by a phage-related integrase. The transferable element, which we called a symbiosis island, represents a novel class of genetic element with similarities to pathogenicity islands that differentiate bacterial pathogens from benign isolates of the same species. We have sequenced the symbiosis island and are now characterising the roles of the 424 genes uncovered. The functional genomics approach we are following involves a combination of mutagenesis and expression analysis using both reporter genes and microarrays. One interesting finding was that the rhizobium translocates specific proteins involved in the symbiosis directly into plant cells. We have also uncovered other acquired "fitness islands" in the genome of the mesorhizobia and are studying the role of these elements in the ecological adaptation of the bacteria. A particular focus is on characterizing the mechanisms by which the islands are transferred, their host range and how their transfer is regulated. This research is providing novel insight into the role of horizontal gene transfer in microbial evolution.

Research in the laboratory is supported by grants from the Marsden Fund, the Foundation for Research, Science and Technology, and AgMardt.

Other Activities

• Chair in Genetics and Director of the Genetics Programme
• Member, University of Otago Institutional Biological Safety Committee

Selected Publications

Sullivan, J.T., Patrick, H.N., Lowther, W.L., Scott, D.B., and Ronson, C.W. (1995) Nodulating strains of Rhizobium loti arise through chromosomal symbiotic gene transfer in the environment. Proceedings of the National Academy of Sciences of the United States of America. 92:8985-8989

Sullivan, J. T. & Ronson, C. W. (1998) Evolution of rhizobia by acquisition of a 500- kb symbiosis island that integrates into a phe-tRNA gene. Proceedings of the National Academy of Sciences of the United States of America 95, 5145-5149

Sullivan, J. T., J. R. Trzebiatowski, R. W. Cruickshank, J. Gouzy, S. D. Brown, R. M. Elliot, D. J. Fleetwood, N. G. McCallum, U. Rossbach, G. S. Stuart, J. E. Weaver, R. J. Webby, F. J. de Bruijn, and C. W. Ronson. (2002). Comparative sequence analysis of the symbiosis island of Mesorhizobium loti strain R7A. Journal of Bacteriology 184:3086-3095

Hubber, A., Vergunst, A. C., Sullivan, J. T., Hooykaas, P. J. and Ronson, C. W. (2004). Symbiotic phenotypes and translocated effector proteins of the Mesorhizobium loti strain R7A VirB/D4 type IV secretion system. Molecular Microbiology 54:561-574