Table 1. Summary of helical symmetry parameters for RecA and RAD51 filaments.
| Structure | Data collection method (resolution) | Pitch in Å | Protomers/turn |
|---|---|---|---|
| RAD51/ssDNA (this report) | EM/Cryo (4.2Å) | 103 | 6.4 |
| RAD51/ssDNA (19) | EM/stain | 99 | 6.39 |
| ScRad51/no DNA (16) | Crystallography (3.25Å) | 130 | 6 |
| ScRad51/dsDNA (19) | EM/stain | 94 | 6.28 |
| RecA (14) | Crystallography (2.3Å) | 82.7 | 6 |
| RecA/ssDNA (12) | Crystallography (2.8Å) | 93.96 | 6.16 |
| RecA/dsDNA (19) | EM/stain | 91 | 6.16 |
| RecA/ssDNA (20) | EM/stain (27Å) | 80 | 6.1 |
The helical symmetry parameter values of the recombinases have been found to vary both between and within structures, depending to some degree as to whether they are unbound, bound to ssDNA or to dsDNA. All structures incorporate 6–6.4 protomers per turn, enabling derivation of helical twist and rise from the values shown. The first entry refers to our RAD51 presynaptic structure reported here. A low-resolution EM structure of human RAD51 by staining has a pitch of 99Å, compared to the 103Å for the high-resolution structure reported here. This difference may be due to duplexing of the DNA in the stain structure; the 99Å pitch reported for this model matches that of a RAD51/dsDNA model that we have recently computed (data not shown). The extended pitch of the ScRad51 filament may stem from the hyper-functional nature of the mutant yeast Rad51 protein and/or the absence of DNA.