Proliferating cell nuclear antigen (PCNA) is responsible for the processivity of DNA polymerase. We determined the crystal structure of Pyrococcus furiosus DNA polymerase (PfuPol) complexed with a cognate monomeric PCNA, which allowed us to construct a convincing model of the polymerase-PCNA ring interaction. Electron microscopy analyses confirmed that this complex structure exists among the multiple functional configurations in solution. Together with data from mutational analyses, this structural study indicated that the novel interaction between a stretched loop of PCNA and the PfuPol Thumb domain is quite important, in addition to the authentic PCNA-polymerase recognition site (PIP box). A comparison of the present structures with the previously reported structures of polymerases complexed with DNA suggested that the second interaction site plays a crucial role in switching between the polymerase and exonuclease modes, by stabilizing only the polymerase mode. This proposed mechanism of fidelity control of replicative DNA polymerases was supported by experiments, in which a mutation within the second interaction site caused an enhancement in the exonuclease activity in the presence of PCNA.
The development of tunable organic fluorophores has met with increasing interest because of their many applications in analytical and material sciences. Recently, we have proposed a tunable solid-state fluorescence system consisted of organic salts of anthracene disulfonic acid (ADS) with primary alkyl amines. The salts were recrystallized from organic solvents to yield pure and suitable crystals for various analyses. The resulting crystals yielded various anthracene arrangements depending on the difference of aliphatic amines. Photophysical properties of the crystals, such as emission spectra, emission quantum efficiencies and life times, are modulated by arrangements of anthracene moieties.
Molecular machines based on the conversion of changes in molecular geometry to mechanical motion in macroscale materials are interesting from the perspective of basic research and offer a wide range of potential applications. Such machines are amenable to remote operation with external stimuli such a photon impingement and do not require direct contact. Azobenzenes are typical chromophores that undergo trans-cis photoisomerization. We have recently found photomechanical bending of trans-4-(dimethylamino)azobenzene microcrystals. Upon UV irradiation, the platelike microcrystal bent quickly in the direction opposite to the light source, reaching the maximum deflection after 0.5 s. The microcrystal returned to its initial flat shape 30 s after the illumination was stopped. This mechanical motion was reversible over repeated 100 cycles of UV irradiation. The bending effect was attributed to a gradient in the extent of UV-induced trans-cis photoisomerization as a function of light penetration, causing the expansion of the irradiated crystal surface to result in a bent macrostructure.
The complex behavior of water and the unusual properties of ferroelectric ice XI continue to attract much interest. Whether ice in the space exists as ice XI, is an important question, because long range electrostatic forces caused by the ferroelectricity might be an important factor for planet formation. From neutron diffraction experiments, we found the temperature and pressure conditions for the transformation of the largest fraction of ice into ferroelectric ice. It suggests that myriad big icy-bodies, which exist as dwarf planets and Kuiper Belt Object, consist of thick ferroelectric-ice surface. Furthermore, we report spectral and vibrational properties of ferroelectric ice investigated by inelastic neutron scattering and infrared absorption measurements. Because the spectral properties of ferroelectric ice are clearly different from those of ordinary ice, the distinct ferroelectric ice in the universe is detectable using infrared telescopes and planetary exploration.