Cloning and characterization of a DEAD box RNA helicase from the viable seedlings of aged mung bean

Seeds stored under adverse conditions will reduce the viability of germination as a result of induced aging. We have established a procedure to induce accelerated aging for studying the process of aging in mung bean (Vigna radiata) seeds at the molecular level. A full-length cDNA was isolated from acceleratedly aged mung bean seedlings. The cDNA, VrRH1 (Vigna radiata RNA helicase 1), contains an open reading frame of 2139 bp encoding a protein of 713 amino acids. VrRH1 has seven highly conserved motifs including the DEAD box as in the case of other plant RNA helicases. VrRH1 was sub-cloned into an expression vector pET-28b (+), over-expressed in Escherichia coli BL 21 and purified by a Ni²⁺-agarose column. The expressed protein showed double-stranded RNA unwinding and ATPase activities. Either ATP or dATP is required for the unwinding activity, indicating that VrRH1 is an ATP/dATP-dependent RNA helicase. Northern blot analysis showed the presence of mRNAs hybridized with a full-length cDNA fragment of VrRH1 (VrRH transcripts) in mung bean seeds that were imbibed for 16 to 32 h after accelerated aging treatment. The amount of these mRNAs reached a maximum in 24 h imbibed seeds after the treatment. The accumulation of VrRH transcripts was shown to lead to the appearance of 25S and 18S rRNAs in the imbibed aging mung bean seeds. The results suggest that VrRH1 may play a role in the viability of mung bean seeds.