Bacterial RecA (Clark & Margulies, 1965
), archaeal RadA (Sandler et al.
) and eukaryal Rad51 (Shinohara et al.
) and DMC1 (Bishop et al.
) proteins form a superfamily of recombinases (also called DNA strand-exchange proteins; Seitz & Kowalczykowski, 2000
). Homologous recombination appears to be essential in the repair of double-stranded DNA breaks and the restarting of stalled replication forks (Cox, 1998
; Cox et al.
; Courcelle et al.
; Lusetti & Cox, 2002
; Kowalczykowski, 2000
). These proteins facilitate a pivotal DNA strand-exchange process between a single-stranded DNA (ssDNA) and a homologous double-stranded DNA (dsDNA) in homologous recombination. Electron-microscopic and crystallographic results have revealed strikingly similar ‘active’ recombinase assemblies in the form of right-handed helical filaments with approximately six monomers per turn (VanLoock et al.
; Conway et al.
; Wu et al.
; Chen et al.
; Sheridan et al.
; Li et al.
). The milestone structures of Escherichia coli
RecA (EcRecA) in complex with a series of DNA molecules have shed light on the exact mechanism of homologous DNA strand exchange (Chen et al.
). Crystallized ‘inactive’ filaments with shorter helical pitches have also been observed (Story et al.
). Ring-shaped forms with 6–8 protomers have also been commonly observed by electron microscopy (Heuser & Griffith, 1989
; Yu & Egelman, 1997
; Passy et al.
; Yang et al.
; Galkin et al.
; McIlwraith et al.
; Masson et al.
). Only heptameric rings of Pyrococcus furiosus
RadA (PfRadA) and octameric Homo sapiens
DMC1 (HsDMC1) have previously been crystallized (Shin et al.
; Kinebuchi et al.
). A reconstructed hexameric EcRecA model has been derived from electron microscopy (Yu & Egelman, 1997
). In addition to the three commonly found forms, crystal structures of overwound three-monomer-per-turn filaments (Ariza et al.
) and left-handed filaments of Sulfolobus solfataricus
RadA (SsRadA; Chen et al.
; Chang et al.
) have also been observed. Here, we report the first crystal structure of hexameric RadA from Methanococcus voltae
devoid of its first 60 amino-acid residues (Δ60
MvRadA). Crystal-packing analysis and comparison with the heptameric PfRadA structure and the octameric HsDMC1 structure indicated that these proteins can form two-ringed assemblies.