Abstract
Abstract DNA helicases catalyse disruption of the hydrogen bonds between the two strands of duplex DNA to generate single-stranded DNA (ssDNA) products (1, 2). This reaction is generally referred to as an unwinding reaction. The ssDNA product(s) of the unwinding reaction is then available for use as a template for DNA replication or repair, or as a substrate in recombination. Enzymes that catalyse the unwinding of duplex RNA, RNA secondary structure, and DNA:RNA hybrids have also been described (3-7). These enzymes have important roles in mRNA biogenesis, transcription, and translation. Thus, helicase catalysed unwinding is not confined to the unwinding of duplex DNA. In fact, there are examples of enzymes that catalyse the un winding of more than one of these substrates (8-10). The well characterized T-antigen, for example, unwinds both duplex DNA and RNA:DNA hybrids (9). Presumably these substrate preferences are related to the biological function of the protein, although these relationships are not always immediately obvious. None the less, it is clear that all transactions involving either DNA or RNA require the (transient) existence of a single-stranded intermediate, and thus helicases play key roles in all aspects of nucleic acid metabolism (11).
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