Chiral supramolecular order revealed during the formation of calf thymus and phage DNA crystals

The control of DNA packaging has been reported to be dependent on an ordered liquid-crystalline state. However, the textural characteristics that are typical of crystals and that resemble mesophases have not been reported for highly polymerized or even shorter types of DNA filaments under in vitro conditions that favor crystallization. Because DNA crystals are expected to exhibit particular textural optical anisotropies, pure and highly polymerized calf thymus DNA and simpler λ phage DNA were crystallized from solution drops and were analyzed using high-performance polarization microscopy with and without differential interference contrast (DIC) optics. Both types of DNA formed crystals that exhibited chiral supramolecular textures resembling the twist-grain boundary (TGB) columnar mesophases described for liquid crystals and exhibited intrinsic negative birefringence. To the best of our knowledge, this is the first observation using polarization/interference optics of pure DNA crystals that have TGB columnar mesophase-like textural characteristics. A comparison of the crystals formed from the highly polymerized calf thymus DNA and those formed from the shorter phage DNA strands revealed textural differences. Compared to the phage DNA crystals, the crystals of highly polymerized thymus DNA exhibited a more intertwisted columnar distribution and a fibrous texture between their columnar structures. In addition, a form birefringence phenomenon was detected only in the thymus DNA crystals. These characteristics are presumed to reflect the higher level of supramolecular order, self-assembly and chirality in highly polymerized calf thymus DNA crystals relative to that of crystals formed from the simpler, shorter, λ phage DNA. The higher-order supramolecular organization revealed here for in vitro DNA preparations raises the possibility that this structure could also occur, possibly to a smaller degree, during DNA self-aggregation under specific in vivo conditions. Whether the DNA crystal properties presently described play a role in the establishment of higher-order levels of hierarchical chromatin structure and consequently in chromatin physiology, should be further investigated.