Photosystem I (PSI) is one of the two photosystems present in oxygenic photosynthetic organisms and functions to harvest and convert light energy into chemical energy in photosynthesis. In eukaryotic algae and higher plants, PSI consists of a core surrounded by variable species and numbers of light-harvesting complex (LHC)I proteins, forming a PSI-LHCI supercomplex. Here, we report cryo-EM structures of PSI-LHCR from the red alga in two forms, one with three Lhcr subunits attached to the side, similar to that of higher plants, and the other with two additional Lhcr subunits attached to the opposite side, indicating an ancient form of PSI-LHCI. Furthermore, the red algal PSI core showed features of both cyanobacterial and higher plant PSI, suggesting an intermediate type during evolution from prokaryotes to eukaryotes. The structure of PsaO, existing in eukaryotic organisms, was identified in the PSI core and binds three chlorophylls and may be important in harvesting energy and in mediating energy transfer from LHCII to the PSI core under state-2 conditions. Individual attaching sites of LHCRs with the core subunits were identified, and each Lhcr was found to contain 11 to 13 chlorophylls and 5 zeaxanthins, which are apparently different from those of LHCs in plant PSI-LHCI. Together, our results reveal unique energy transfer pathways different from those of higher plant PSI-LHCI, its adaptation to the changing environment, and the possible changes of PSI-LHCI during evolution from prokaryotes to eukaryotes.

SummaryGrass carp reovirus (GCRV) is a member of the Aquareovirus genus of the family Reoviridae, a large family of dsRNA viruses infecting plants, insects, fishes and mammals. We report the first subnanometer-resolution three-dimensional (3D) structures of both GCRV core and virion by cryoelectron microscopy (cryoEM). These structures have allowed the delineation of interactions among the over 1000 molecules in this enormous macromolecular machine, and a detail comparison with other dsRNA viruses at the secondary structure level. The GCRV core structure shows that the inner proteins have strong structural similarities even at the level of secondary structure elements with those of orthoreoviruses, indicating that the structures involved in viral dsRNA interaction and transcription are highly conserved. In contrast, the level of similarity in structures decreases in the proteins situated in the outer layers of the virion. The proteins involved in host recognition and attachment exhibit the least similarities to other members of Reoviridae. Furthermore, in GCRV, the RNA-translocating turrets are in an open state and lack a counterpart for the σ1 protein situated on top of the close turrets observed in mammalian orthoreovirus (MRV). Interestingly, the distribution and organization of GCRV core proteins resembles those of the cytoplasmic polyhedrosis virus (CPV), a cypovirus and the structurally simplest member of the Reoviridae family. Our results suggest that GCRV occupies a unique structure niche between the simpler cypoviruses and the considerably more complex MRV, thus providing an important model for understanding the structural and functional conservation and diversity of this enormous family of dsRNA viruses.

Double-stranded RNA (dsRNA) viruses possess a segmented dsRNA genome and a number of RNA-dependent RNA polymerases (RdRps) enclosed in a capsid. Until now, the precise structures of genomes and RdRps within the capsids have been unknown. Here we report the structures of RdRps and associated RNAs within nontranscribing and transcribing cypoviruses (NCPV and TCPV, respectively), using a combination of cryo-electron microscopy (cryo-EM) and a symmetry-mismatch reconstruction method. The RdRps and associated RNAs appear to exhibit a pseudo-D3 symmetric organization in both NCPV and TCPV. However, the molecular interactions between RdRps and the genomic RNA were found to differ in these states. Our work provides insight into the mechanisms of the replication and transcription in dsRNA viruses and paves a way for structural determination of lower-symmetry complexes enclosed in higher-symmetry structures.

Grass carp reovirus (GCRV) is a member of the aquareovirus genus in the Reoviridae family and has a capsid with two shells—a transcription-competent core surrounded by a coat. We report a near-atomic-resolution reconstruction of the GCRV virion by cryo-electron microscopy and single-particle reconstruction. A backbone model of the GCRV virion, including seven conformers of the five capsid proteins making up the 1500 molecules in both the core and the coat, was derived using cryo-electron microscopy density-map-constrained homology modeling and refinement. Our structure clearly showed that the amino-terminal segment of core protein VP3B forms an ~120-Å-long α-helix-rich extension bridging across the icosahedral 2-fold-symmetry-related molecular interface. The presence of this unique structure across this interface and the lack of an external cementing molecule at this location in GCRV suggest a stabilizing role of this extended amino-terminal density. Moreover, part of this amino-terminal extension becomes invisible in the reconstruction of transcription-competent core particles, suggesting its involvement in endogenous viral RNA transcription. Our structure of the VP1 turret represents its open state, and comparison with its related structures at the closed state suggests hinge-like domain movements associated with the mRNA-capping machinery. Overall, this first backbone model of an aquareovirus virion provides a wealth of structural information for understanding the structural basis of GCRV assembly and transcription.

The cytoplasmic polyhedrosis virus (CPV) from the family Reoviridae belongs to a subgroup of "turreted" reoviruses, in which the mRNA capping activity occurs in a pentameric turret. We report a full atomic model of CPV built from a 3D density map obtained using cryoelectron microscopy. The image data for the 3D reconstruction were acquired exclusively from a CCD camera. Our structure shows that the enzymatic domains of the pentameric turret of CPV are topologically conserved and that there are five unique channels connecting the guanylyltransferase and methyltransferase regions. This structural organization reveals how the channels guide nascent mRNA sequentially to guanylyltransferase, 7-Nmethyltransferase, and 2′-O-methyltransferase in the turret, undergoing the highly coordinated mRNA capping activity. Furthermore, by fitting the deduced amino acid sequence of the protein VP5 to 120 large protrusion proteins on the CPV capsid shell, we confirmed that this protrusion protein is encoded by CPV RNA segment 7.V iruses of the family Reoviridae have a segmented dsRNA genome enclosed by single, double, or triple capsid shells. They share a similar transcription mechanism in which the inner capsids (core) remain intact and serve as shelters protecting the transcriptional process from antiviral defense mechanisms inside the cytoplasm of the host cell during replication of their dsRNA genomes. Despite the absence of significant sequence homology among different genera of the Reoviridae, the innermost capsid shells, which are formed by two conformers of the one protein, of all members of the Reoviridae and the majority of dsRNA viruses share common functions (1).The "turreted" reoviruses have been defined as a subgroup of the Reoviridae due to their similar protein composition and capsid architecture. For this subgroup of reoviruses, a pentameric turret formed by five copies of turret proteins on fivefold vertex of the innermost shell functions in the catalysis of mRNA 5′ cap synthesis (2-5). Cryo-EM or crystal structures are available for four turreted virus genera of the family Reoviridae: the Cypovirus (6), Orthoreovirus (2), Oryzavirus (7), and Aquareovirus (8, 9) genera. These viruses have a segmented genome consisting of 10-12 linear segments of dsRNA and their hosts include vertebrates (orthoreoviruses and aquareovriuses), invertebrates (cypoviruses), and plants (oryzaviruses).The cytoplasmic polyhedrosis virus (CPV) belongs to the genus Cypovirus and has a dsRNA genome of 10 segments. It is unique among dsRNA viruses in having a single capsid layer (10), which corresponds to the core of orthoreoviruses and functions as a stable mRNA synthesis machine in the cytoplasm of host cells that transcribes mRNAs from the segmented double-stranded RNA templates (11). CPV virions are embedded in polyhedra capable of surviving dehydration, freezing, and chemical treatments that would denature most proteins (12). The polyhedra dissolve in the alkaline midgut of their hosts and release infectious virions during the infection ...

S U M M A R YRecent variations of the ground surface temperature, recorded in the Earth's subsurface, can be inverted from borehole temperature data. The resolution of the inversion of borehole temperature logs is poor, even when the noise level is low. This report concerns the potential improvement in the resolution of ground surface temperature history when temperature logs from several boreholes are inverted simultaneously. For an inversion algorithm based on singular value decomposition, the singular values obtained for simultaneous inversion of several boreholes do not decrease as rapidly as the singular values for a single borehole. With the same value of the cut-off or of the damping parameter, more eigenvectors remain in the solution and give higher resolution for several boreholes than for one. Tests conducted with synthetic data for 15 boreholes show that the improvement in resolution is real but not spectacular. These tests indicate that borehole temperature logs should have approximately the same sampling interval. Temperature profiles over different depth ranges can be inverted if the reference heat flows and ground temperatures are included in the parameters determined by the inversion. Tests also show that no consistent ground-temperature history (GTH) is obtained when inverting data from boreholes that have experienced different surfacetemperature variations. Simultaneous inversion can be applied to: (1) obtain a local GTH for a single site with several boreholes having identical surface conditions, or (2) obtain regional averages with data from different sites that have experienced the same variations in ground temperature.Temperature profiles measured in four boreholes near Belleterre, in eastern Canada, appear to have recorded the same perturbation. Inversion of the temperature data from individual boreholes yields similar, but not identical, GTHs. The GTH obtained by simultaneous inversion of these four boreholes indicates a cool period, with minimum temperatures at ca. 1800 AD, followed by warming above the reference level. It is consistent with other analyses indicating recent warming in eastern Canada.

Most double-stranded RNA (dsRNA) viruses transcribe RNA plus strands within a common innermost capsid shell. This process requires coordinated efforts by RNA-dependent RNA polymerase (RdRp) together with other capsid proteins and genomic RNA. Here we report the near-atomic resolution structure of the RdRp protein VP2 in complex with its cofactor protein VP4 and genomic RNA within an aquareovirus capsid using 200-kV cryoelectron microscopy and symmetry-mismatch reconstruction. The structure of these capsid proteins enabled us to observe the elaborate nonicosahedral structure within the double-layered icosahedral capsid. Our structure shows that the RdRp complex is anchored at the inner surface of the capsid shell and interacts with genomic dsRNA and four of the five asymmetrically arranged N termini of the capsid shell proteins under the fivefold axis, implying roles for these N termini in virus assembly. The binding site of the RNA end at VP2 is different from the RNA cap binding site identified in the crystal structure of orthoreovirus RdRp λ3, although the structures of VP2 and λ3 are almost identical. A loop, which was thought to separate the RNA template and transcript, interacts with an apical domain of the capsid shell protein, suggesting a mechanism for regulating RdRp replication and transcription. A conserved nucleoside triphosphate binding site was localized in our RdRp cofactor protein VP4 structure, and interactions between the VP4 and the genomic RNA were identified.

Double-stranded RNA viruses in the family Reoviridae are capable of transcribing and capping nascent mRNA within an icosahedral viral capsid that remains intact throughout repeated transcription cycles. However, how the highly coordinated mRNA transcription and capping process is facilitated by viral capsid proteins is still unknown. Cypovirus provides a good model system for studying the mRNA transcription and capping mechanism of viruses in the family Reoviridae . Here, we report a full backbone model of a transcribing cypovirus built from a near-atomic-resolution density map by cryoelectron microscopy. Compared with the structure of a nontranscribing cypovirus, the major capsid proteins of transcribing cypovirus undergo a series of conformational changes, giving rise to structural changes in the capsid shell: ( i ) an enlarged capsid chamber, which provides genomic RNA with more flexibility to move within the densely packed capsid, and ( ii ) a widened peripentonal channel in the capsid shell, which we confirmed to be a pathway for nascent mRNA. A rod-like structure attributable to a partially resolved nascent mRNA was observed in this channel. In addition, conformational change in the turret protein results in a relatively open turret at each fivefold axis. A GMP moiety, which is transferred to 5’-diphosphorylated mRNA during the mRNA capping reaction, was identified in the pocket-like guanylyltransferase domain of the turret protein.