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NodA PROTEIN SEQUENCES AS PREDICTORS OF THE LIPOCHITO-OLIGOSACCHARIDE Nod FACTOR TYPE: A TOOL FOR STUDYING THE MOLECULAR ECOLOGY AND EVOLUTION OF RHIZOBIUM-LEGUME SYMBIOSIS
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L. Moulin , B. Mangin , F. Debelle , E. Giraud , S. Ba , J. Denarie , C. Boivin-Masson
!LSTM, IRD-INRA-CIRAD-ENSAM, Baillarguet, BP 5035- 34398 Montpellier Cedex 5, France
2BIA, INRA, BP 27, 31326 Castanet-Tolosan Cedex, France
3LBMRPM, CNRS-INRA, BP 27, 31326 Castanet-Tolosan Cedex, France
The nodulation gene nodA, which is present in a single copy in all rhizobia, is involved in the specific transfer of an acyl chain to the chito-oligosaccharide backbone of Nod factors (NFs) (Debelle et al. 1996; Ritsema et al. 1996; Quinto et al. 1997; Schultze et al. 1995).
We sequenced the entire nodA gene of rhizobial strains, whose NFs have been characterized and which belong to several different genera. Phylogenetic analysis of all the available sequences, indicated that the NodA proteins form clusters correlating with some NF structural features such as fucosylation and/or arabinosylation at the reducing end, and the presence of polyunsaturated fatty acids at the non-reducing end. We proposed two complementary methods to predict the NF type of a given strain. One based on the complete NodA sequences phylogenetical analysis and the other based on the analysis of amino acids present at NF-type informative positions.
These tools were used to obtain clues on the NF structure of 30 strains including novel species or symbionts with a peculiar host range. Twenty-four predictions were in accordance with available biological or biochemical data. In some cases, including new described rhizobia Methylobacterium nodulans and Burkholderia sp. STM678 (Sy et al. 2001; Moulin et al. 2001) no safe prediction could be proposed, suggesting that the strains produced new NF structures.
These results indicate that the nodA gene could be used as a tool to search for novel NF structures and as markers for studying rhizobium-legume coevolution.
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