Mutations in two locations occupied by leucines, for example, inhibit self-interaction between the Mre11 proteins and the formation of the U-shaped dimer; as a result, Mre11's ability to bind to DNA is reduced and interactions with the other MRN proteins, Rad50 and Nbs1, can be affected.
On the other hand, mutations at two histidine sites do not affect the ability of Mre11 to form dimers or interact with Rad50, Nbs1, or other repair proteins, but do affect its ability to remodel DNA. One mutation, which affects the remodeling of DNA ends, causes fewer problems. The other histidine mutation impairs Mre11's ability to remodel both end and internal sequences and subsequently cripples repair. Because MRN initiates only one kind of DSB repair, however homologous recombination in some species a different kind of repair called nonhomologous end-joining can take up the slack.
The right tools for the job
DNA was bound to Mre11 dimers in the correct orientation and crystallized at the Scripps Research Institute in La Jolla, CA. Critical for the results was the analysis of the samples performed by the Scripps Institute's Williams at the SIBYLS beamline at Berkeley Lab's Advanced Light Source.
The SIBYLS beamline, 12.3.1, has unique capabilities to analyze the shape of samples in solution using small-angle x-ray scattering (SAXS), as well as details of samples in crystals using macromolecular crystallography (MX). Scott Classen of Berkeley Lab's Physical Biosciences Division (PBD) he
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DOE/Lawrence Berkeley National Laboratory