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Fusarium graminearum is the causal agent of a serious scab disease of small grains in Korea. We screened 827 isolates of F. graminearum from diseased barley and maize and tested for the presence of double-stranded RNA (dsRNA) mycovirus. Of them, 19 isolates contained various sizes of dsRNAs. A dsRNA associated with pronounced morphological changes including reduction in mycelial growth, increase in dark orange to red pigmentation, reduced sporulation and virulence was previously observed in nine dsRNA-containing Fusarium isolates (Chu et al., Appl Env Microbiol 68, 2529-2534, 2002). Ten additional isolates were found infected with dsRNA mycoviruses. These mycoviruses contain 2-4 different segments of dsRNAs with the size-range of approximately 1.7-10 kbp in length. The presence of dsRNAs did not affect colony morphology and were transmissible through conidia and ascospore with incidence of 30-100%. Interestingly, dsRNA mycovirus found in F. graminearum isolates, JB33 and JNKY19, that show the pattern of mixed infection of two different viruses were transmitted to all progeny conidia and ascospores. These results indicate that there is genomic diversity of dsRNA mycoviruses that infect F. graminearum isolates and that impact of virus infection on host's morphology and virulence is determined by the interaction between dsRNAs and the fungal host, not by the mere presence of the dsRNAs.

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The potyvirus coat protein (CP) is involved in aphid transmission, cell-to-cell movement and virus assembly, not only by binding to viral RNA, but also by self-interaction or interactions with other factors. In this study, a number of CP mutants of Soybean mosaic virus (SMV) containing deletions and site-directed mutations were generated and cloned into yeast two-hybrid vectors. Interaction was confirmed by the expression of reporter genes, including HIS3, ADE2 and MEL1, in yeast strain AH109. Deletion of the C-terminal region of the CP caused loss of the CP-CP self-interaction ability detected in CP mutants with the C-terminal region. Alanine substitution at the amino acid positions R190, E191, E212, R245, H246 and R249 disrupted CP-CP interaction, whereas substitutions at the amino acid positions R188, D189, D198, K205, K218 and D250 did not. These results indicate that the C-terminal region of SMV CP may contain a domain(s) or amino acids required for CP-CP interaction and virus assembly.Diseases caused by Soybean mosaic virus (SMV) have been reported in many countries and have significantly reduced crop yields and quality worldwide (Jayaram et al., 1992;Hill, 1999). Despite the economic importance of SMV, relatively little is understood about the detailed molecular interactions between viral proteins and the host proteins that occur during viral infections. The viral RNA-dependent RNA polymerase (nuclear inclusion protein b) of Tobacco vein mottling virus reportedly interacts with its own genomelinked viral protein (VPg) and coat protein (CP) (Hong et al., 1995;Li et al., 1997). An interaction between the CP and the helper-component protease (HC-Pro) of Zucchini yellow mosaic virus has also been reported (Peng et al., 1998). The nuclear inclusion protein a (NIa) and cylindrical inclusion proteins have also been reported to interact with each other (Guo et al., 2001). Such interactions may be important for virus replication, virus movement and insect transmission (Blanc et al., 1997;Rojas et al., 1997;Carrington et al., 1998). Recently, we applied the yeast two-hybrid system (YTHS) to identify protein interactions among ten viral proteins generated from the SMV polyprotein (Kang et al., 2004). Here, we used the YTHS to identify key amino acid sequences or domains responsible for the self-interaction of SMV CP.Four deletion mutations were introduced into SMV-G7H CP (Fig. 1a) to define regions required for CP self-interaction. Truncated clones of the CP were fused downstream of both GAL4-DBD (pAS2-1) and GAL4-AD (pACT2; Clontech). The yeast two-hybrid plasmids pAS2-1 and pACT2, host strain AH109 (MATa, gal4D, gal80D), all media, buffers and methods for the yeast two-hybrid assay were adapted from the Matchmaker System 2 (Clontech). Proteins expressed by yeast cells were extracted (Yeast Protocol Handbook, Clontech) and verified by using SDS-PAGE followed by immunoblot with GAL4 antibodies (1 : 250 dilution; Santa Cruz Biotechnology) using an ECL kit (Amersham Biosciences). All truncated CP mutants were detecte...

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