Nonlinear grassland responses to past and future atmospheric CO(2)

Carbon sequestration in soil organic matter may moderate increases in atmospheric CO(2) concentrations (C(a)) as C(a) increases to more than 500 micromol mol(-1) this century from interglacial levels of less than 200 micromol mol(-1) (refs 1 6). However, such carbon storage depends on feedbacks between plant responses to C(a) and nutrient availability. Here we present evidence that soil carbon storage and nitrogen cycling in a grassland ecosystem are much more responsive to increases in past C(a) than to those forecast for the coming century. Along a continuous gradient of 200 to 550 micromol mol(-1) (refs 9, 10), increased C(a) promoted higher photosynthetic rates and altered plant tissue chemistry. Soil carbon was lost at subambient C(a), but was unchanged at elevated C(a) where losses of old soil carbon offset increases in new carbon. Along the experimental gradient in C(a) there was a nonlinear, threefold decrease in nitrogen availability. The differences in sensitivity of carbon storage to historical and future C(a) and increased nutrient limitation suggest that the passive sequestration of carbon in soils may have been important historically, but the ability of soils to continue as sinks is limited.     

Endogenous cannabinoids mediate retrograde signalling at hippocampal synapses

Marijuana affects brain function primarily by activating the G-protein-coupled cannabinoid receptor-1 (CB1), which is expressed throughout the brain at high levels. Two endogenous lipids, anandamide and 2-arachidonylglycerol (2-AG), have been identified as CB1 ligands. Depolarized hippocampal neurons rapidly release both anandamide and 2-AG in a Ca2+-dependent manner. In the hippocampus, CB1 is expressed mainly by GABA (gamma-aminobutyric acid)-mediated inhibitory interneurons, where CB1 clusters on the axon terminal. A synthetic CB1 agonist depresses GABA release from hippocampal slices. These findings indicate that the function of endogenous cannabinoids released by depolarized hippocampal neurons might be to downregulate GABA release. Here we show that the transient suppression of GABA-mediated transmission that follows depolarization of hippocampal pyramidal neurons is mediated by retrograde signalling through release of endogenous cannabinoids. Signalling by the endocannabinoid system thus represents a mechanism by which neurons can communicate backwards across synapses to modulate their inputs.     

Genome sequence of enterohaemorrhagic Escherichia coli O157:H7

The bacterium Escherichia coli O157:H7 is a worldwide threat to public health and has been implicated in many outbreaks of haemorrhagic colitis, some of which included fatalities caused by haemolytic uraemic syndrome. Close to 75,000 cases of O157:H7 infection are now estimated to occur annually in the United States. The severity of disease, the lack of effective treatment and the potential for large-scale outbreaks from contaminated food supplies have propelled intensive research on the pathogenesis and detection of E. coli O157:H7 (ref. 4). Here we have sequenced the genome of E. coli O157:H7 to identify candidate genes responsible for pathogenesis, to develop better methods of strain detection and to advance our understanding of the evolution of E. coli, through comparison with the genome of the non-pathogenic laboratory strain E. coli K-12 (ref. 5). We find that lateral gene transfer is far more extensive than previously anticipated. In fact, 1,387 new genes encoded in strain-specific clusters of diverse sizes were found in O157:H7. These include candidate virulence factors, alternative metabolic capacities, several prophages and other new functions--all of which could be targets for surveillance.     

The physical maps for sequencing human chromosomes 1, 6, 9, 10, 13, 20 and X

We constructed maps for eight chromosomes (1, 6, 9, 10, 13, 20, X and (previously) 22), representing one-third of the genome, by building landmark maps, isolating bacterial clones and assembling contigs. By this approach, we could establish the long-range organization of the maps early in the project, and all contig extension, gap closure and problem-solving was simplified by containment within local regions. The maps currently represent more than 94% of the euchromatic (gene-containing) regions of these chromosomes in 176 contigs, and contain 96% of the chromosome-specific markers in the human gene map. By measuring the remaining gaps, we can assess chromosome length and coverage in sequenced clones.     

The genetic basis of family conflict resolution in mice

Asymmetries in the costs and benefits of parental investment for mothers, fathers and offspring result in family conflict over the production and provisioning of young. In species where females provide most resources before and after birth, the resolution of this conflict may be influenced by genes expressed in mothers and by maternally and paternally inherited genes expressed in offspring. Here we disentangle these effects by means of reciprocal mating and cross-fostering of litters between two strains of mice that differ with respect to the typical resolution of family conflict. We find that differences in litter size between these two strains are determined by paternal genotype, whereas differences in provisioning are under maternal control, showing that there is antagonistic coadaptation of maternal and paternal effects on distinct life-history traits. Maternal provisioning is also influenced by the type of foster offspring. Contradictory to theoretical expectations, however, we find no evidence for a negative correlation across strains between maternal provisioning and offspring demand. Instead, we show that there is positive coadaptation such that offspring obtain more resources from foster mothers of the same strain as their natural mother, irrespective of their father's strain.     

A uniquely specialized ear in a very early tetrapod

The Late Devonian genus Ichthyostega was for many decades the earliest known tetrapod, and the sole representative of a transitional form between a fish and a land vertebrate. However, despite being known since 1932 (ref. 1) from a large collection of specimens, its morphology remained enigmatic and not what was expected of a very primitive tetrapod. Its apparent specializations led it to be considered as a "blind offshoot" or "sidebranch" off the tetrapod family tree, and recent cladistic analyses have disagreed about its exact phylogenetic position within the tetrapod stem group. In particular, its braincase and ear region defied interpretation, such that conventional anatomical terms seemed inapplicable. Using new material collected in 1998 (ref. 9), preparation of earlier-collected material, and high-resolution computed tomography scanning, here we identify and interpret these problematic anatomical structures. They can now be seen to form part of a highly specialized ear, probably a hearing device for use in water. This represents a structurally and functionally unique modification of the tetrapod otic region, unlike anything seen in subsequent tetrapod evolution. The presence of deeply grooved gill bars as in its contemporary Acanthostega suggest that Ichthyostega may have been more aquatically adapted than previously believed.     

A cryo-electron microscopic study of ribosome-bound termination factor RF2

Protein synthesis takes place on the ribosome, where genetic information carried by messenger RNA is translated into a sequence of amino acids. This process is terminated when a stop codon moves into the ribosomal decoding centre (DC) and is recognized by a class-1 release factor (RF). RFs have a conserved GGQ amino-acid motif, which is crucial for peptide release and is believed to interact directly with the peptidyl-transferase centre (PTC) of the 50S ribosomal subunit. Another conserved motif of RFs (SPF in RF2) has been proposed to interact directly with stop codons in the DC of the 30S subunit. The distance between the DC and PTC is approximately 73 A. However, in the X-ray structure of RF2, SPF and GGQ are only 23 A apart, indicating that they cannot be at DC and PTC simultaneously. Here we show that RF2 is in an open conformation when bound to the ribosome, allowing GGQ to reach the PTC while still allowing SPF-stop-codon interaction. The results indicate new interpretations of accuracy in termination, and have implications for how the presence of a stop codon in the DC is signalled to PTC.