TARGET-SITE SPECIFICITY OF TRANSPOSITION OF THE RICE RETROTRANSPOSON Tos17 ACTIVATED BY TISSUE CULTURE

National Institute of Agrobiological Resources, Tsukuba, Ibaraki 305-8602, Japan

Some of the retrotransposons in the plant genome can be activated by tissue culture. In rice, one of the endogenous retrotransposons Tos17 is highly activated and frequently integrates into the genes. Therefore, it can be used for gene tagging. We are now constructing the rice mutant-panel, the large population of regenerated rice plant lines that carry several transposed copies of Tos17 per line. To evaluate the effectiveness of using such a mutant-panel for functional analysis of genes, we analyzed the target-site specificity of Tos17 transposition. Over 100 sequences flanking transposed Tos17 copies were isolated from 550 regenerated lines carrying about 8000 transposed copies and analyzed. A part of them were mapped onto the genetic map. Target loci were widely spread over the rice genome. Multiple independent integration events were observed in several genes. Furthermore, numerous independent integrations into Tos17 itself were observed. These results suggest the presence of hot spots of Tos17 integration, whereas specific sequence motifs of the integration target were not observed. Because most of the target sequences were non-redundant, saturation mutagenesis with Tos17 should be feasible.