A NOVEL RAS-RELATED RGP1 GENE ENCODING A GTP-BINDING PROTEIN HAS REDUCED EXPRESSION IN 5-AZACYTIDINE-INDUCED DWARF RICE

Exposure of normal, tall rice (Oryza sativa) seedlings to 5-azacytidine, a powerful inhibitor of DNA methylation in vivo, induced both demethylation of genomic DNA and dwarf plants. Genes that had been affected by treatment were identified by differential screening of a cDNA library, and a ras-related gene, rgp1, was subsequently isolated. The cDNA of rgp1 was found to encode a deduced protein sequence of 226 amino acids with a relative molecular mass of 24850, which was most closely related to the ras-related ypt3 protein of fission yeast, Shizosaccharomyces pombe. The rgp1 protein, expressed in transformed Escherichia coli, clearly showed GTP-binding activity. During seedling growth, rgp1 expression was first observed 14 days after germination, reaching a maximum level between 28 and 42 days, and gradually decreased thereafter until 63 days when it attained the same level of expression as in 14-day-old seedlings. Expression of rgp1 was found to be markedly reduced throughout the growth period of both 5-azacytidine-induced dwarf plants and their progenies, relative to levels in untreated tall control plants. These results suggest that expression of rgp1 may be influenced, either directly or indirectly, by DNA methylation, and that the rgp1 protein may play an important role in plant growth and development.