The modal-Hamiltonian interpretation and the Galilean covariance of quantum mechanics

The aim of this paper is to analyze the modal-Hamiltonian interpretation of quantum mechanics in the light of the Galilean group. In particular, it is shown that the rule of definite-value assignment proposed by that interpretation has the same properties of Galilean covariance and invariance as the Schrödinger equation. Moreover, it is argued that, when the Schrödinger equation is invariant, the rule can be reformulated in an explicitly invariant form in terms of the Casimir operators of the Galilean group. Finally, the possibility of extrapolating the rule to quantum field theory is considered.     

Following the signal sequence from ribosomal tunnel exit to signal recognition particle

Membrane and secretory proteins can be co-translationally inserted into or translocated across the membrane. This process is dependent on signal sequence recognition on the ribosome by the signal recognition particle (SRP), which results in targeting of the ribosome-nascent-chain complex to the protein-conducting channel at the membrane. Here we present an ensemble of structures at subnanometre resolution, revealing the signal sequence both at the ribosomal tunnel exit and in the bacterial and eukaryotic ribosome-SRP complexes. Molecular details of signal sequence interaction in both prokaryotic and eukaryotic complexes were obtained by fitting high-resolution molecular models. The signal sequence is presented at the ribosomal tunnel exit in an exposed position ready for accommodation in the hydrophobic groove of the rearranged SRP54 M domain. Upon ribosome binding, the SRP54 NG domain also undergoes a conformational rearrangement, priming it for the subsequent docking reaction with the NG domain of the SRP receptor. These findings provide the structural basis for improving our understanding of the early steps of co-translational protein sorting.     

Insights from the genome of the biotrophic fungal plant pathogen Ustilago maydis

Ustilago maydis is a ubiquitous pathogen of maize and a well-established model organism for the study of plant-microbe interactions. This basidiomycete fungus does not use aggressive virulence strategies to kill its host. U. maydis belongs to the group of biotrophic parasites (the smuts) that depend on living tissue for proliferation and development. Here we report the genome sequence for a member of this economically important group of biotrophic fungi. The 20.5-million-base U. maydis genome assembly contains 6,902 predicted protein-encoding genes and lacks pathogenicity signatures found in the genomes of aggressive pathogenic fungi, for example a battery of cell-wall-degrading enzymes. However, we detected unexpected genomic features responsible for the pathogenicity of this organism. Specifically, we found 12 clusters of genes encoding small secreted proteins with unknown function. A significant fraction of these genes exists in small gene families. Expression analysis showed that most of the genes contained in these clusters are regulated together and induced in infected tissue. Deletion of individual clusters altered the virulence of U. maydis in five cases, ranging from a complete lack of symptoms to hypervirulence. Despite years of research into the mechanism of pathogenicity in U. maydis, no 'true' virulence factors had been previously identified. Thus, the discovery of the secreted protein gene clusters and the functional demonstration of their decisive role in the infection process illuminate previously unknown mechanisms of pathogenicity operating in biotrophic fungi. Genomic analysis is, similarly, likely to open up new avenues for the discovery of virulence determinants in other pathogens.     

The inner-nuclear-envelope protein emerin regulates HIV-1 infectivity

Primate lentiviruses such as human immunodeficiency type 1 (HIV-1) have the capacity to infect non-dividing cells such as tissue macrophages. In the process, viral complementary DNA traverses the nuclear envelope to integrate within chromatin. Given the intimate association between chromatin and the nuclear envelope, we examined whether HIV-1 appropriates nuclear envelope components during infection. Here we show that emerin, an integral inner-nuclear-envelope protein, is necessary for HIV-1 infection. Infection of primary macrophages lacking emerin was abortive in that viral cDNA localized to the nucleus but integration into chromatin was inefficient, and conversion of viral cDNA to non-functional episomal cDNA increased. HIV-1 cDNA associated with emerin in vivo, and the interaction of viral cDNA with chromatin was dependent on emerin. Barrier-to-autointegration factor (BAF), the LEM (LAP, emerin, MAN) binding partner of emerin, was required for the association of viral cDNA with emerin and for the ability of emerin to support virus infection. Therefore emerin, which bridges the interface between the inner nuclear envelope and chromatin, may be necessary for chromatin engagement by viral cDNA before integration.     

Transiting extrasolar planetary candidates in the Galactic bulge

More than 200 extrasolar planets have been discovered around relatively nearby stars, primarily through the Doppler line shifts owing to reflex motions of their host stars, and more recently through transits of some planets across the faces of the host stars. The detection of planets with the shortest known periods, 1.2-2.5 days, has mainly resulted from transit surveys which have generally targeted stars more massive than 0.75 M(o), where M(o) is the mass of the Sun. Here we report the results from a planetary transit search performed in a rich stellar field towards the Galactic bulge. We discovered 16 candidates with orbital periods between 0.4 and 4.2 days, five of which orbit stars of masses in the range 0.44-0.75 M(o). In two cases, radial-velocity measurements support the planetary nature of the companions. Five candidates have orbital periods below 1.0 day, constituting a new class of ultra-short-period planets, which occur only around stars of less than 0.88 M(o). This indicates that those orbiting very close to more-luminous stars might be evaporatively destroyed or that jovian planets around stars of lower mass might migrate to smaller radii.     

Transient cross-reactive immune responses can orchestrate antigenic variation in malaria

The malaria parasite Plasmodium falciparum has evolved to prolong its duration of infection by antigenic variation of a major immune target on the surface of the infected red blood cell. This immune evasion strategy depends on the sequential, rather than simultaneous, appearance of immunologically distinct variants. Although the molecular mechanisms by which a single organism switches between variants are known in part, it remains unclear how an entire population of parasites within the host can synchronize expression to avoid rapidly exhausting the variant repertoire. Here we show that short-lived, partially cross-reactive immune responses to parasite-infected erythrocyte surface antigens can produce a cascade of sequentially dominant antigenic variants, each of which is the most immunologically distinct from its preceding types. This model reconciles several previously unexplained and apparently conflicting epidemiological observations by demonstrating that individuals with stronger cross-reactive immune responses can, paradoxically, be more likely to sustain chronic infections. Antigenic variation has always been seen as an adaptation of the parasite to evade host defence: we show that the coordination necessary for the success of this strategy might be provided by the host.     

Compact sources as the origin of the soft gamma-ray emission of the Milky Way

The Milky Way is known to be an abundant source of gamma-ray photons, now determined to be mainly diffuse in nature and resulting from interstellar processes. In the soft gamma-ray domain, point sources are expected to dominate, but the lack of sensitive high-resolution observations did not allow for a clear estimate of the contribution from such sources. Even the best imaging experiment revealed only a few point sources, accounting for about 50% of the total Galactic flux. Theoretical studies were unable to explain the remaining intense diffuse emission. Investigating the origin of the soft gamma-rays is therefore necessary to determine the dominant particle acceleration processes and to gain insights into the physical and chemical equilibrium of the interstellar medium. Here we report observations in the soft gamma-ray domain that reveal numerous compact sources. We show that these sources account for the entirety of the Milky Way's emission in soft gamma-rays, leaving at most a minor role for diffuse processes.     

Genetic evidence that relative synaptic efficacy biases the outcome of synaptic competition

Synaptic activity drives synaptic rearrangement in the vertebrate nervous system; indeed, this appears to be a main way in which experience shapes neural connectivity. One rearrangement that occurs in many parts of the nervous system during early postnatal life is a competitive process called 'synapse elimination'. At the neuromuscular junction, where synapse elimination has been analysed in detail, muscle fibres are initially innervated by multiple axons, then all but one are withdrawn and the 'winner' enlarges. In support of the idea that synapse elimination is activity dependent, it is slowed or speeded when total neuromuscular activity is decreased or increased, respectively. However, most hypotheses about synaptic rearrangement postulate that change depends less on total activity than on the relative activity of the competitors. Intuitively, it seems that the input best able to excite its postsynaptic target would be most likely to win the competition, but some theories and results make other predictions. Here we use a genetic method to selectively inhibit neurotransmission from one of two inputs to a single target cell. We show that more powerful inputs are strongly favoured competitors during synapse elimination.