Abstract
Virulent variants of Rice yellow mottle virus (genus Sobemovirus) can emerge on the highly resistant rice cultivars Gigante and Bekarosaka. Non-synonymous mutations responsible of the breakdown of the recessive resistant gene rymv1-2 were located in the VPg after determination of its termini in the polyprotein P2a. The secondary structure of this protein was predicted to include a central -helix. The two major amino acids related to virulence are located in the same side of this helix. The 3D-topology and the biochemical properties of virulence mutations both suggested a direct site-to-site interaction between RYMV VPg and rice eIF(iso)4G encoded by rymv1. Rice yellow mottle virus (RYMV) of the genus Sobemovirus [7] is present in all rice-growing African countries where it causes high yield losses [9]. Two Oryza sativa indica cultivars Gigante and Bekarosaka and a few O. glaberrima cultivars Togs expressed high resistance towards RYMV [1]. However, several RYMV isolates of different strains were able to overcome the high resistance [13]. Mutations associated with virulence towards cvs. Gigante and Bekarosaka were restricted to five sites in the RYMV genome [13]. They are located in a stretch of 15 amino acids in the polyprotein P2a which contains sequence motifs of the serine protease and of the VPg (Viral Protein genome-linked). The sites and the kinetics of RYMV polyprotein maturation are still unknown. However, the proximity of the virulence sites to the triplet WAD which is conserved in VPg sobemoviruses [11] suggested that they belong to the putative VPg domain. Moreover, VPgs are involved in potyvirus virulence