The GTPase EF-Tu in ternary complex with GTP and aminoacyl-tRNA (aa-tRNA) promotes rapid and accurate delivery of cognate aa-tRNAs to the ribosomal A site. Here we used cryo-EM to study the molecular origins of the accuracy of ribosome-aided recognition of a cognate ternary complex and the accuracy-amplifying role of the monitoring bases A1492, A1493 and G530 of the 16S rRNA. We used the GTPase-deficient EF-Tu variant H84A with native GTP, rather than non-cleavable GTP analogues, to trap a near-cognate ternary complex in high-resolution ribosomal complexes of varying codon-recognition accuracy. We found that ribosome complexes trapped by GTPase-deficicent ternary complex due to the presence of EF-TuH84A or non-cleavable GTP analogues have very similar structures. We further discuss speed and accuracy of initial aa-tRNA selection in terms of conformational changes of aa-tRNA and stepwise activation of the monitoring bases at the decoding center of the ribosome.

SUMMARYIncreasing the tissue biomass and/or volume of the gastrointestinal tract (GIT) is commonly seen when animals feed on poorquality diets. This increase can simply permit larger meal sizes, but may also rebalance nutritionally imbalanced ingesta by allowing selective absorption of limiting nutrients. In an insect herbivore, the migratory locust, a synthetic diet with a high ratio of protein to carbohydrate was found to induce mass enhancement of the GIT. When normalised for sex and overall body size, increases to the mass of the foregut and midgut caeca resulted in higher absorption (20-30%) of both protein and carbohydrate when subsequently feeding on three chemically and structurally different grasses. Greater net absorption of macronutrients occurred because these locusts ate larger meals that transited at the same time and with the same digestive efficiency as locusts in which the GIT was not enlarged. Thus, plasticity of the GIT did not improve nutritional homeostasis, but increased the rate of nutrient uptake. Supplementary material available online at

Visual recognition of conspecifics is necessary for a wide range of social behaviours in many animals. Medaka (Japanese rice fish), a commonly used model organism, are known to be attracted by the biological motion of conspecifics. However, biological motion is a composite of both body-shape motion and entire-field motion trajectory (i.e., posture or motion-trajectory elements, respectively), and it has not been revealed which element mediates the attractiveness. Here, we show that either posture or motion-trajectory elements alone can attract medaka. We decomposed biological motion of the medaka into the two elements and synthesized visual stimuli that contain both, either, or none of the two elements. We found that medaka were attracted by visual stimuli that contain at least one of the two elements. In the context of other known static visual information regarding the medaka, the potential multiplicity of information regarding conspecific recognition has further accumulated. Our strategy of decomposing biological motion into these partial elements is applicable to other animals, and further studies using this technique will enhance the basic understanding of visual recognition of conspecifics.

SUMMARYHepatitis C virus mRNA contains an internal ribosome entry site (IRES) that mediates end-independent translation initiation, requiring a subset of eukaryotic initiation factors (eIFs). Direct binding of the IRES to the 40S subunit places the initiation codon into the P site, where it base-pairs with eIF2-bound Met-tRNAiMet forming a 48S initiation complex. Then, eIF5 and eIF5B mediate subunit joining. Initiation can also proceed without eIF2, in which case Met-tRNAiMet is recruited directly by eIF5B. Here, we present cryo-EM structures of IRES initiation complexes at resolutions up to 3.5 Å that cover all major stages from initial ribosomal association, through eIF2-containing 48S initiation complexes, to eIF5B-containing complexes immediately prior to subunit joining. These structures provide insights into the dynamic network of 40S/IRES contacts, highlight the role for IRES domain II, and reveal conformational changes that occur during the transition from eIF2- to eIF5B-containing 48S complexes that prepare them for subunit joining.

Hepatitis C virus mRNA contains an internal ribosome entry site (IRES) that mediates end‐independent translation initiation, requiring a subset of eukaryotic initiation factors (eIFs). Biochemical studies revealed that direct binding of the IRES to the 40S ribosomal subunit places the initiation codon into the P site, where it base pairs with eIF2‐bound Met‐tRNAiMet forming a 48S initiation complex. Subsequently, eIF5 and eIF5B mediate subunit joining, yielding an elongation‐competent 80S ribosome. Initiation can also proceed without eIF2, in which case Met‐tRNAiMet is recruited directly by eIF5B. However, the structures of initiation complexes assembled on the HCV IRES, the transitions between different states, and the accompanying conformational changes have remained unknown. To fill these gaps, we now obtained cryo‐EM structures of IRES initiation complexes, at resolutions up to 3.5 Å, that cover all major stages from the initial ribosomal association, through eIF2‐containing 48S initiation complexes, to eIF5B‐containing complexes immediately prior to subunit joining. These structures provide insights into the dynamic network of 40S/IRES contacts, highlight the role of IRES domain II, and reveal conformational changes that occur during the transition from eIF2‐ to eIF5B‐containing 48S complexes and prepare them for subunit joining.