A permineralized species of mangrove fern Acrostichum L. from Deccan Intertrappean beds of India

Acrostichum intertrappeum sp. nov., a permineralized aerial stem with helically arranged petioles and roots in organic connection, is described from the Deccan Intertrappean beds of Nawargaon, District Wardha, Maharashtra, India and forms the first and the oldest record of its kind. The stem is covered with a thick ramentum of large multicellular scales and is characterized by a three angled dictyostele enclosing a few medullary bundles. The petioles are helically arranged and the vascular morphology is distinct with abaxial horseshoe-shaped ring, adaxial linear row and central dorso-ventrally elongated ring. The roots have hexarch steles and aerenchymatous cortex with large air cavities in rings. Occurrence of Acrostichum along with coastal palms, mangroves and marsh plants described earlier from this region and nearby locations suggests the north-western shore of the Deccan was linked to the equatorial ocean (South Western Tethys Sea) probably through the Narmada Valley during the deposition of Intertrappean sediments. Morphological features and anatomical characters of the fossil reveal that Acrostichum grew in marsh swamp environment or on mud flats of back water areas of the coastal environment prevailing a tropical humid climate when the Deccan region was almost at an equatorial position during the Late Cretaceous period. This new record further adds to a growing body of data on the diversification of polypodiaceous ferns that point to a much earlier crown group radiation of the group than previously thought.     

Structural basis for interaction of O-acetylserine sulfhydrylase and serine acetyltransferase in the Arabidopsis cysteine synthase complex

In plants, association of O-acetylserine sulfhydrylase (OASS) and Ser acetyltransferase (SAT) into the Cys synthase complex plays a regulatory role in sulfur assimilation and Cys biosynthesis. We determined the crystal structure of Arabidopsis thaliana OASS (At-OASS) bound with a peptide corresponding to the C-terminal 10 residues of Arabidopsis SAT (C10 peptide) at 2.9-A resolution. Hydrogen bonding interactions with key active site residues (Thr-74, Ser-75, and Gln-147) lock the C10 peptide in the binding site. C10 peptide binding blocks access to OASS catalytic residues, explaining how complex formation downregulates OASS activity. Comparison with bacterial OASS suggests that structural plasticity in the active site allows binding of SAT C termini with dissimilar sequences at structurally similar OASS active sites. Calorimetric analysis of the effect of active site mutations (T74S, S75A, S75T, and Q147A) demonstrates that these residues are important for C10 peptide binding and that changes at these positions disrupt communication between active sites in the homodimeric enzyme. We also demonstrate that the C-terminal Ile of the C10 peptide is required for molecular recognition by At-OASS. These results provide new insights into the molecular mechanism underlying formation of the Cys synthase complex and provide a structural basis for the biochemical regulation of Cys biosynthesis in plants.     

An unexpected sequence of events: mismatch detection in the human hippocampus

The ability to identify and react to novelty within the environment is fundamental to survival. Computational models emphasize the potential role of the hippocampus in novelty detection, its unique anatomical circuitry making it ideally suited to act as a comparator between past and present experience. The hippocampus, therefore, is viewed to detect associative mismatches between what is expected based on retrieval of past experience and current sensory input. However, direct evidence that the human hippocampus performs such operations is lacking. We explored brain responses to novel sequences of objects using functional magnetic resonance imaging (fMRI), while subjects performed an incidental target detection task. Our results demonstrate that hippocampal activation was maximal when prior predictions concerning which object would appear next in a sequence were violated by sensory reality. In so doing, we establish the biological reality of associative match-mismatch computations within the human hippocampus, a process widely held to play a cardinal role in novelty detection. Our results also suggest that the hippocampus may generate predictions about how future events will unfold, and critically detect when these expectancies are violated, even when task demands do not require it. The present study also offers broader insights into the nature of essential computations carried out by the hippocampus, which may also underpin its unique contribution to episodic memory.     

Crystal structure of phosphatidylglycerophosphatase (PGPase), a putative membrane-bound lipid phosphatase, reveals a novel binuclear metal binding site and two "proton wires"

Phosphatidylglycerophosphatase (PGPase), an enzyme involved in lipid metabolism, catalyzes formation of phosphatidylglycerol from phosphatidylglycerophosphate. Phosphatidylglycerol is a multifunctional phospholipid, found in the biological membranes of many organisms. Here, we report the crystal structure of Listeria monocytogenes PGPase at 1.8 A resolution. PGPase, an all-helical molecule, forms a homotetramer. Each protomer contains an independent active site with two metal ions, Ca(2+) and Mg(2+), forming a hetero-binuclear center located in a hydrophilic cavity near the surface of the molecule. The binuclear center, conserved ligands, metal-bound water molecules, and an Asp-His dyad form the active site. The catalytic mechanism of this enzyme is likely to proceed via binuclear metal activated nucleophilic water. The binuclear metal-binding active-site environment of this structure should provide insights into substrate binding and metal-dependent catalysis. A long channel with inter-linked linear water chains, termed "proton wires," is observed at the tetramer interface. Comparison of similar water chain structures in photosynthetic reaction centers (RCs), Cytochrome f, gramicidin, and bacteriorhodopsin, suggests that PGPase may conduct protons via proton wires.     

The dynamics of hippocampal activation during encoding of overlapping sequences

Sequence disambiguation, the process by which overlapping sequences are kept separate, has been proposed to underlie a wide range of memory capacities supported by the hippocampus, including episodic memory and spatial navigation. We used functional magnetic resonance imaging (fMRI) to explore the dynamic pattern of hippocampal activation during the encoding of sequences of faces. Activation in right posterior hippocampus, only during the encoding of overlapping sequences but not nonoverlapping sequences, was found to correlate robustly with a subject-specific behavioral index of sequence learning. Moreover, our data indicate that hippocampal activation in response to elements common to both sequences in the overlapping sequence pair, may be particularly important for accurate sequence encoding and retrieval. Together, these findings support the conclusion that the human hippocampus is involved in the earliest stage of sequence disambiguation, when memory representations are in the process of being created, and provide empirical support for contemporary computational models of hippocampal function.     

Structure- and substrate-based inhibitor design for Clostridium botulinum neurotoxin serotype A

The seven antigenically distinct serotypes of Clostridium botulinum neurotoxins cleave specific soluble N-ethylmaleimide-sensitive factor attachment protein receptor complex proteins and block the release of neurotransmitters that cause flaccid paralysis and are considered potential bioweapons. Botulinum neurotoxin type A is the most potent among the clostridial neurotoxins, and to date there is no post-exposure therapeutic intervention available. To develop inhibitors leading to drug design, it is imperative that critical interactions between the enzyme and the substrate near the active site are known. Although enzyme-substrate interactions at exosites away from the active site are mapped in detail for botulinum neurotoxin type A, information about the active site interactions is lacking. Here, we present the crystal structures of botulinum neurotoxin type A catalytic domain in complex with four inhibitory substrate analog tetrapeptides, viz. RRGC, RRGL, RRGI, and RRGM at resolutions of 1.6-1.8 A. These structures show for the first time the interactions between the substrate and enzyme at the active site and delineate residues important for substrate stabilization and catalytic activity. We show that OH of Tyr(366) and NH(2) of Arg(363) are hydrogen-bonded to carbonyl oxygens of P1 and P1' of the substrate analog and position it for catalytic activity. Most importantly, the nucleophilic water is replaced by the amino group of the N-terminal residue of the tetrapeptide. Furthermore, the S1' site is formed by Phe(194), Thr(215), Thr(220), Asp(370), and Arg(363). The K(i) of the best inhibitory tetrapeptide is 157 nm.     

Within-plant and within-leaf dispersion pattern of two-spotted spider mite,Tetranychus urticae Koch (Acari: Tetranychidae) on okra

Tetranychus urticae distribution on spatial scale both within leaf and within plant was assessed by the index of dispersion (Id), Lloyd's mean crowding index (Imc), Lloyd's patchiness index (Ip) and Morisita's index (I _δ). Id values in all leaves and leaf parts promised aggregated distribution regardless of leaf position and leaf areas. The values of Imc estimated were all larger than the mean. The value of Ip and I _δ  also indicated increased degree of aggregation and clumping in all leaves and leaf areas. While pattern of distribution is same in all the leaves and leaf parts, population estimated is high in young fully opened top leaves than the grown-up middle and bottom leaves. Within the young leaf, top area of upper side of the leaf housed relatively increased number of T. urticae than bottom area of upper side of the leaf. Because young fully opened leaves consistently contained major proportion of mites (48.33%), particularly the top leaf area (79.62%), these leaves and leaf areas can be used as sampling unit for population estimation to minimise the time spent on sampling. However, it warrants future research for predictive models to associate a number of mites of top leaf to other leaves and top area to other areas. Further, releasing predators in young fully opened leaves may increase predator efficiency if supplementary studies on predator–prey relationship on spatial scale are triggered.