ABSTRACT
Helicases are a family of enzymes essential to many aspects of DNA metabolism, including DNA replication, recombination, transcription, and repair and, for RNA translation, splicing and ribosomal assembly. Comparison of amino acid sequences suggests that helicases can be classified into two superfamilies, SF1 and SF2, and a smaller family consisting of DnaB-like helices (5). In the latter family, E. coli DnaB, RuvB, Rho, RepA, and bacteriophage T7 gp4 and T4 gp41 are helicases with a hexameric ring structure (6). This family of helicases is defined by five conserved regions. The E. coli DnaB and the bacteriophage T7 gp4 are the
most extensively studied replicative helicases that catalyze strand separation at the replication fork. Both are 5 0-3 0 helicases which exhibit a conformational flexibility. The hexameric DnaB has been shown to exist in two different forms, with a threefold symmetry (a trimer of dimers), or sixfold symmetry, and a range of intermediate states (7). The T7 gp4 also likely involves stable dimer, trimer, and higher oligomer species (8). The functional implication of these conformations is not clear. The T7 gp4 protein, unlike DnaB, is bifunctional, consisting of a helicase domain and a primase domain, collectively known as helicase-primase.
