Modularity and Specialization in Superfamily 1 and 2 Helicases

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Abstract
Helicases may be classified in various ways. A common functional distinction is to group them by directionality. A given length of NA duplex is essentially symmetric, possessing dyad symmetry (ignoring base composition); thus, helicase polarity was originally specified by referring to the direction of the flanking single-strand (ss) region of NA usually required to initiate unwinding. Thus, 3′-5′ helicases require a 3′ tail on the substrate duplex while a 5′ tail is required for the 5′-3′ enzymes. It should be noted, however, that some helicases are capable of initiating unwinding from a blunt-ended duplex, such as the RecBCD system in E. coli (45, 57). In view of this, it is more appropriate to describe the directionality in terms of the strand upon which the enzyme translocates, a definition that is consistent with, but expands upon, the previously described system.