Propagation strategy of the double-stranded RNA replicon symbiotically in a rice cell.

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We have found various double-stranded RNAs(dsRNAs) from many species of crops and wild plants. The International Committee on Taxonomy of Viruses(ICTV) recently accepted Endornavirus as a new genus, consisting of large (about 14 to 17 kbp) plasmid-like dsRNA replions in rice and broad bean. However, the sequence information of endornaviruses is limited and only four dsRNAs have been sequenced entirely. Partial nucleotide sequences of RNA-dependent RNA polymerase regions were obtained from large dsRNAs in barley (Hordeum vulgare), kidney bean (Phaseolus vulgaris), melon (Cucumis melo), bottle gourd (Lagenaria siceraria), Malabar spinach (Basella alba), seagrass (Zostera marina), and the fungus Helicobasidium mompa. Phylogenetic analyses of these seven dsRNAs with known endornaviruses and single-stranded RNA viruses indicate that these dsRNAs are new members of Endornavirus that are widely distributed over plants and fungi. The phylogenetic relationships between these dsRNAs and their ho … More st plants suggest that ancestors of dsRNA replicons were horizontally transmissible.We compared total protein contents between dsRNA-carrier and dsRNA-free rice cultured cells and detected some differences. These results suggest that a host protein, which can regulate the dsRNA replicons, should exist in a plant cell. We presumed that the host proteins could bind dsRNA specifically and this protein should have a conserved dsRNA-binding motif (dsRBM). We characterized biochemically ten Arabidopsis dsRBM-containing proteins. Two Dicer-like proteins (DCL1 and DCL3), HYL1/DRB1 and the three HYL1 homologs exhibit significant dsRNA-binding activity. Furthermore, we show that DRB4 interacts specifically with DCL4,and HYL1 most strongly interacts with DCL1. These results indicate that each HYL1/DRB family protein interacts with one specific partner among the four Dicer-like proteins. The presented data suggest that each member of the HYL1/DRB protein family may individually modulate Dicer function through heterodimerization with a Dicer-like protein in vivo. Less