Plant DNA helicases: the long unwinding road

DNA helicases are molecular motor proteins that use the energy of nucleoside 5'-triphosphate (NTP) hydrolysis to open transiently the energetically stable duplex DNA into single strands and thereby play essential roles in nearly all DNA metabolic transactions. After the discovery of the first prokaryotic DNA helicase from E. coli in 1976 and the first eukaryotic one from the lily plant in 1978, many more have been isolated and characterized including at least eight from plants. All the DNA helicases share some common properties, including nucleic acid binding, NTP binding and hydrolysis and unwinding of duplex DNA in the 3' to 5' or 5' to 3' direction. In plants, DNA helicases are mainly present in nuclei, mitochondria and chloroplasts. The in vivo role of many DNA helicases has not been well investigated in eukaryotic systems including plants. However, through indirect evidence, the involvement of plant DNA helicases has been suggested at least in the following biological processes: DNA recombination, DNA replication, translation initiation, rDNA transcription and in the early stages of pre-rRNA processing, double-strand break repair, maintenance of telomeric length, nucleotide excision repair, cell division/proliferation during flower development, maintenance of genomic methylation patterns, the plant cell cycle, and in the maintenance of the basic activities of cells. A recently discovered Helitron insertion in the maize genome has suggested the possible role of plant DNA helicase(s) in a new class of rolling-circle transposons. All these reflect that plant DNA helicases may play an important role in plant growth and development and thus have important biotechnological applications. In this review, an up-to-date knowledge of plant DNA helicases is summarized. In addition, the historical perspective, biochemical assay and polarity, inhibitors and functions of plant DNA helicases have also been covered.