Fig. 1: MR oligomerizes via head domain and forms nucleolytically active assemblies on DNA.

a TEM of S. cerevisiae Mre11-Rad50 dimers (M₂R₂) and oligomers (M₂R₂)_n with open (i, ii) or closed (iii) Rad50 coiled-coils. MR oligomerization via head domains (arrows in ii, iii; gray cartoon lines in iii, iv) and further interaction through coiled-coil tethering (arrows in iv). (v) Larger MR oligomers at 30 °C with co-factors ATPγS, Mg²⁺, and Mn²⁺ (Supplementary Fig. 1a–c). Images representative of n = 3. b TEM of MR binding to plasmid DNA at 30 °C with co-factors as above. (i) M₂R₂ binding the DNA end. (ii, iii) MR dimers oligomerizing on DNA and some MR molecules forming “pearls-on-a-string”-like structures (zoom-insets of examples with cartoons), irrespective of DNA ends (Supplementary Fig. 1e, f). n = 3. c EMSA of MR binding to plasmid DNA (i) and quantification (ii; Supplementary Fig. 1g). Mean ± SEM, n = 3 or n = 4 independent experiments, sigmoidal fit. d (i) Resection assay with probe (green box in cartoon) binding to the 3′-overhang produced. n = 2. (ii) TEM of (i) without pSae2 but with ATPγS to stabilize MR on DNA. n = 3. Large MR oligomers still catalyze resection with pSae2 and ATP. e Quantification of EMSAs as in Supplementary Fig. 2a with oligonucleotides of different lengths (100 nM base pairs). K_D as MR concentration at 50% DNA binding ± SEM of n_20,31bp = 4, n_70,100,197bp = 3, unpaired two-tailed t-test. f Molecular mass distributions from mass photometry show better MR^wt binding to longer oligonucleotides (100 nM base pairs). Cartoons illustrate species in peaks. Measured molecular weight (MW, kDa) ± SD in gray. Zoom insets show MW range where a peak of two MR dimers bound to one DNA molecule would be expected and was fitted where possible (Supplementary Fig. 3c). n = 3 independent experiments. g Model illustrating that in the presence of DNA, MR oligomers dissociate to dimers which then oligomerize again via head domain interactions at DNA ends or internally to “pearls-on-a-string”-like assemblies and larger clusters. Substrate compaction suggests DNA loop formation and coiled-coil tethering. Scale bars: 100 nm (a, b, d).