RNA Helicases
RNA helicases and DNA helicases can be found together in all the helicase superfamilies except for SF6. However, not all RNA helicases exhibit helicase activity as defined by enzymatic function, i.e., proteins of the Swi/Snf family. Although these proteins carry the typical helicase motifs, hydrolize ATP in a nucleic acid-dependent manner, and are built around a helicase core, in general, no unwinding activity is observed.
RNA helicases that do exhibit unwinding activity have been characterized by at least two different mechanisms: canonical duplex unwinding and local strand separation. Canonical duplex unwinding is the stepwise directional separation of a duplex strand, as described above, for DNA unwinding. However, local strand separation occurs by a process wherein the helicase enzyme is loaded at any place along the duplex. This is usually aided by a single-stranded region of the RNA, and the loading of the enzyme is accompanied with ATP binding. Once the helicase and ATP are bound, local strand separation occurs, which requires binding of ATP but not the actual process of ATP hydrolysis. Presented with fewer base pairs the duplex then dissociates without further assistance from the enzyme. This mode of unwinding is used by DEAD-box helicases.
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