Abstract
Homologous recombination is a universal mechanism for repairing DNA double strand breaks (DSBs) and injured DNA replication forks. RecA family proteins play a central role in homologous recombination by forming nucleoprotein filaments. These proteins include the prokaryotic RecA, archaeal RadA, and eukaryotic Rad51 and Dmc1. Here we report three crystal structures of the Sulfolobus solfataricus RadA. All of these structures are packed in the orthorhombic lattice and form the left-handed helical filaments with different helical pitches. The results strongly suggest the universal existence of left-handed helical filaments of the RecA family proteins. The structural variations of alternate protein-protein interfaces not only exhibit the extreme flexibility of filaments but also demonstrate the asymmetry within the filaments, implying the sequential binding of DNA and hydrolysis of ATP by RecA family proteins. Further structural analyses and subsequent mutagenesis studies, coupled with biochemical assays, offer a novel insight into the mechanism controlling the SsoRadA quaternary structures and filament assemblies during the catalysis of strand exchange, which involves Asp 70 and Arg 72 in the N-terminus of the NTD-CTD hinge.
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