SummaryPhotosystem II (PSII) is a huge membrane-protein complex consisting of 20 different subunits with a total molecular mass of 350 kDa for a monomer, and catalyzes light-driven water oxidation at its catalytic center, the oxygen-evolving complex (OEC) [1][2][3] . The structure of PSII has been analyzed at 1.9 Å resolution by synchrotron radiation X-rays, which revealed that OEC is a Mn4CaO5 cluster organized in an asymmetric, "distorted-chair" form 4 . This structure was further analyzed with femtosecond X-ray free electron lasers (XFEL), providing the "radiation damage-free" 5 structure. The mechanism of O=O bond formation, however, remains obscure due to the lack of intermediate state structures. Here we report the structural changes of PSII induced by 2-flash (2F) illumination at room temperature at a resolution of 2.35 Å using time-resolved serial femtosecond crystallography (TR-SFX) with an XFEL provided by the SPring-8 angstrom compact free-electron laser (SACLA). Isomorphous differenceFourier map between the 2F and dark-adapted states revealed two areas of apparent changes; they are around QB/non-heme iron and the Mn4CaO5 cluster. The changes around the QB/non-heme iron region reflected the electron and proton transfers induced by the 2F-illumination. In the region around the Mn4CaO5 cluster, a water molecule located 3.5 Å from the Mn4CaO5 cluster disappeared from the map upon 2Fillumination, leading to a closer distance between another water molecule and O4, suggesting also the occurrence of proton transfer. Importantly, the 2F-dark isomorphous difference Fourier map showed an apparent positive peak around O5, a unique μ3-oxo-bridge located in the quasi-center of Mn1 and Mn4 4,5 . This suggests an insertion of a new oxygen atom (O6) close to O5, providing an O=O distance of 1.5 Å between these two oxygen atoms. This provides a mechanism for the O=O bond formation 4 consistent with that proposed by Siegbahn 6,7 . Fig. 1a shows organization of the electron transfer chain of PSII in a pseudo-C2 symmetry by two subunits D1 and D2. The water-oxidation reaction proceeds via the Si-state cycle 8 (with i=0-4), where dioxygen is produced in the transition of S3→(S4)→S0 (Fig. 1b). The high-resolution structures of PSII analyzed so far were for the dark-stable S1 state 4,5 , although a few studies on the low-resolution intermediate S-state structures have been reported by TR-SFX [9][10][11] . During the revision of our manuscript, Young et al. reported a 2F-illuminated state structure at 2.25 Å resolution where no apparent changes around O5 were observed 12 , although estimations of the resolution could yield somewhat different values so that small movement of some water molecules may escape the detection. In order to achieve resolution high enough to uncover small structural changes induced by flash illuminations yet allowing Si-state transition to proceed efficiently, we determined the optimal crystal size of PSII with a maximum length of 100 µm, which diffracted up to a resolution of 2.1 Å by a SACLA-XFEL ...

The structure of the yeast L-A virus was determined by X-ray crystallography at 3.4 A resolution. The L-A dsRNA virus is 400 A in diameter and contains a single protein shell of 60 asymmetric dimers of the coat protein, a feature common among the inner protein shells of dsRNA viruses and probably related to their unique mode of transcription and replication. The two identical subunits in each dimer are in non-equivalent environments and show substantially different conformations in specific surface regions. The L-A virus decaps cellular mRNA to efficiently translate its own uncapped mRNA. Our structure reveals a trench at the active site of the decapping reaction and suggests a role for nearby residues in the reaction.

Rice dwarf virus (RDV), the causal agent of rice dwarf disease, is a member of the genus Phytoreovirus in the family Reoviridae. RDV is a double-shelled virus with a molecular mass of approximately 70 million Dalton. This virus is widely prevalent and is one of the viruses that cause the most economic damage in many Asian countries. The atomic structure of RDV was determined at 3.5 A resolution by X-ray crystallography. The double-shelled structure consists of two different proteins, the core protein P3 and the outer shell protein P8. The atomic structure shows structural and electrostatic complementarities between both homologous (P3-P3 and P8-P8) and heterologous (P3-P8) interactions, as well as overall conformational changes found in P3-P3 dimer caused by the insertion of amino-terminal loop regions of one of the P3 protein into the other. These interactions suggest how the 900 protein components are built into a higher-ordered virus core structure.

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Nanoscale imaging of biological specimens in their native condition is of long-standing interest, in particular with direct, high resolution views of internal structures of intact specimens, though as yet progress has been limited. Here we introduce wet coherent x-ray diffraction microscopy capable of imaging fully hydrated and unstained biological specimens. Whole cell morphologies and internal structures better than 25 nm can be clearly visualized without contrast degradation.