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Open Access Research article

Genome sequencing of two Neorhizobium galegae strains reveals a noeT gene responsible for the unusual acetylation of the nodulation factors

Janina Österman135, Joanne Marsh6, Pia K Laine2, Zhen Zeng1, Edward Alatalo2, John T Sullivan4, J Peter W Young5, Jane Thomas-Oates67, Lars Paulin2 and Kristina Lindström13*

Author Affiliations

1 Department of Food and Environmental Sciences, University of Helsinki, Viikinkaari 9, 00790 Helsinki, Finland

2 Institute of Biotechnology, University of Helsinki, Viikinkaari 9, 00790 Helsinki, Finland

3 Department of Environmental Sciences, University of Helsinki, Viikinkaari 2a, 00790 Helsinki, Finland

4 Department of Microbiology and Immunology, University of Otago, Dunedin 9054, New Zealand

5 Department of Biology, University of York, Heslington, York YO10 5DD, UK

6 Department of Chemistry, University of York, Heslington, York YO10 5DD, UK

7 Centre of Excellence in Mass Spectrometry, University of York, Heslington, York YO10 5DD, UK

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BMC Genomics 2014, 15:500  doi:10.1186/1471-2164-15-500

Published: 19 June 2014

Abstract

Background

The species Neorhizobium galegae comprises two symbiovars that induce nodules on Galega plants. Strains of both symbiovars, orientalis and officinalis, induce nodules on the same plant species, but fix nitrogen only in their own host species. The mechanism behind this strict host specificity is not yet known. In this study, genome sequences of representatives of the two symbiovars were produced, providing new material for studying properties of N. galegae, with a special interest in genomic differences that may play a role in host specificity.

Results

The genome sequences confirmed that the two representative strains are much alike at a whole-genome level. Analysis of orthologous genes showed that N. galegae has a higher number of orthologs shared with Rhizobium than with Agrobacterium. The symbiosis plasmid of strain HAMBI 1141 was shown to transfer by conjugation under optimal conditions. In addition, both sequenced strains have an acetyltransferase gene which was shown to modify the Nod factor on the residue adjacent to the non-reducing-terminal residue. The working hypothesis that this gene is of major importance in directing host specificity of N. galegae could not, however, be confirmed.

Conclusions

Strains of N. galegae have many genes differentiating them from strains of Agrobacterium, Rhizobium and Sinorhizobium. However, the mechanism behind their ecological difference is not evident. Although the final determinant for the strict host specificity of N. galegae remains to be identified, the gene responsible for the species-specific acetylation of the Nod factors was identified in this study. We propose the name noeT for this gene to reflect its role in symbiosis.

Keywords:
Neorhizobium galegae; Genome; Nod factor; noeT; Orthologs; Mass spectrometry; Conjugative plasmid