A DNA helicase from Pisum sativum is homologous to translation initiation factor and stimulates topoisomerase I activity

DNA helicases play an essential role in all aspects of nucleic acid metabol ism, by providing a duplex-unwinding function. This is the first report of the isolation of a cDNA (1.6 kb) clone encoding functional DNA helicase fro m a plant (pea, Pisum sativum). The deduced amino-acid sequence has eight c onserved helicase motifs of the DEAD-box protein family. It is a unique mem ber of this family, containing DESD and SRT motifs instead of DEAD/H and SA T. The encoded 45.5 kDa protein has been overexpressed in bacteria and puri fied to homogeneity. The purified protein contains ATP-dependent DNA and RN A helicase, DNA-dependent ATPase, and ATP-binding activities. The protein s equence contains striking homology with eIF-4A, which has not so far been r eported as DNA helicase. The antibodies against pea helicase inhibit in vit ro translation. The gene is expressed as 1.6 kb mRNA in different organs of pea. The enzyme is localized in the nucleus and cytosol, and unwinds DNA i n the 3' to 5' direction. The pea helicase interacts with pea topoisomerase I protein and stimulates its activity. These results suggest that pea DNA helicase could be an important multifunctional protein involved in protein synthesis, maintaining the basic activities of the cell, and in upregulatio n of topoisomerase I activity. The discovery of such a protein with intrins ic multiple activity should make an important contribution to our better un derstanding of DNA and RNA transactions in plants.