Sequence-programmable covalent bonding of designed DNA assemblies

Bottom-up fabrication of custom nanostructures using the methods of DNA nanotechnology has great potential for applications in many areas of science and technology. One obstacle to applications concerns the constrained environmental conditions at which DNA objects retain their structure. We present a general, site-selective, and scalable method for creating additional covalent bonds that increase the structural stability of DNA nanostructures. Placement of thymidines in close proximity within DNA nanostructures allows the rational creation of sites for covalent cyclobutane pyrimidine dimer (CPD) bonds induced via ultraviolet irradiation. The additional covalent bonds may be used in a sequence-programmable fashion to link free strand termini, to bridge strand breaks at crossover sites, and to create additional interhelical connections. Thus designed multilayer DNA origami objects can remain stable at temperatures up to 90°C and in pure double-distilled water with no additional cations present. In addition, these objects show enhanced resistance against nuclease activity. Cryo–electron microscopy (cryo-EM) structural analysis of non–cross-linked and cross-linked objects indicated that the global shape and the internal network of crossovers are preserved after irradiation. A cryo-EM map of a CPD-stabilized multilayer DNA origami object determined at physiological ionic strength reveals a substantial swelling behavior, presumably caused by repulsive electrostatic forces that, without covalent stabilization, would cause disassembly at low ionic strength. Our method opens new avenues for applications of DNA nanostructures in a wider range of conditions.