Molecular hydrogen beyond the optical edge of an isolated spiral galaxy

Knowledge about the outermost portions of galaxies is limited owing to the small amount of light coming from them. It is known that in many cases atomic hydrogen (H I) extends well beyond the optical radius. In the centres of galaxies, however, molecular hydrogen (H2) usually dominates by a large factor, raising the question of whether H2 is also abundant in the outer regions. Here we report the detection of emission from carbon monoxide (CO), the most abundant tracer of H2, beyond the optical radius of the nearby galaxy NGC 4414. The host molecular clouds probably formed in the regions of relatively high H i column density and in the absence of spiral density waves. The relative strength of the lines from the two lowest rotational levels indicates that both the temperature and density of the H2 are quite low compared to conditions closer to the centre. The inferred surface density of the molecular material continues the monotonic decrease from the inner regions. We conclude that although molecular clouds can form in the outer region of this galaxy, there is little mass associated with them.     

Insights into assembly from structural analysis of bacteriophage PRD1

The structure of the membrane-containing bacteriophage PRD1 has been determined by X-ray crystallography at about 4 A resolution. Here we describe the structure and location of proteins P3, P16, P30 and P31. Different structural proteins seem to have specialist roles in controlling virus assembly. The linearly extended P30 appears to nucleate the formation of the icosahedral facets (composed of trimers of the major capsid protein, P3) and acts as a molecular tape-measure, defining the size of the virus and cementing the facets together. Pentamers of P31 form the vertex base, interlocking with subunits of P3 and interacting with the membrane protein P16. The architectural similarities with adenovirus and one of the largest known virus particles PBCV-1 support the notion that the mechanism of assembly of PRD1 is scaleable and applies across the major viral lineage formed by these viruses.     

Optically programmable electron spin memory using semiconductor quantum dots

The spin of a single electron subject to a static magnetic field provides a natural two-level system that is suitable for use as a quantum bit, the fundamental logical unit in a quantum computer. Semiconductor quantum dots fabricated by strain driven self-assembly are particularly attractive for the realization of spin quantum bits, as they can be controllably positioned, electronically coupled and embedded into active devices. It has been predicted that the atomic-like electronic structure of such quantum dots suppresses coupling of the spin to the solid-state quantum dot environment, thus protecting the 'spin' quantum information against decoherence. Here we demonstrate a single electron spin memory device in which the electron spin can be programmed by frequency selective optical excitation. We use the device to prepare single electron spins in semiconductor quantum dots with a well defined orientation, and directly measure the intrinsic spin flip time and its dependence on magnetic field. A very long spin lifetime is obtained, with a lower limit of about 20 milliseconds at a magnetic field of 4 tesla and at 1 kelvin.     

Membrane structure and interactions with protein and DNA in bacteriophage PRD1

Membranes are essential for selectively controlling the passage of molecules in and out of cells and mediating the response of cells to their environment. Biological membranes and their associated proteins present considerable difficulties for structural analysis. Although enveloped viruses have been imaged at about 9 A resolution by cryo-electron microscopy and image reconstruction, no detailed crystallographic structure of a membrane system has been described. The structure of the bacteriophage PRD1 particle, determined by X-ray crystallography at about 4 A resolution, allows the first detailed analysis of a membrane-containing virus. The architecture of the viral capsid and its implications for virus assembly are presented in the accompanying paper. Here we show that the electron density also reveals the icosahedral lipid bilayer, beneath the protein capsid, enveloping the viral DNA. The viral membrane contains about 26,000 lipid molecules asymmetrically distributed between the membrane leaflets. The inner leaflet is composed predominantly of zwitterionic phosphatidylethanolamine molecules, facilitating a very close interaction with the viral DNA, which we estimate to be packaged to a pressure of about 45 atm, factors that are likely to be important during membrane-mediated DNA translocation into the host cell. In contrast, the outer leaflet is enriched in phosphatidylglycerol and cardiolipin, which show a marked lateral segregation within the icosahedral asymmetric unit. In addition, the lipid headgroups show a surprising degree of order.     

Immune recognition of a human renal cancer antigen through post-translational protein splicing

Cytotoxic T lymphocytes (CTLs) detect and destroy cells displaying class I molecules of the major histocompatibility complex (MHC) that present oligopeptides derived from aberrant self or foreign proteins. Most class I peptide ligands are created from proteins that are degraded by proteasomes and transported, by the transporter associated with antigen processing, from the cytosol into the endoplasmic reticulum, where peptides bind MHC class I molecules and are conveyed to the cell surface. C2 CTLs, cloned from human CTLs infiltrating a renal cell carcinoma, kill cancer cells overexpressing fibroblast growth factor-5 (FGF-5). Here we show that C2 cells recognize human leukocyte antigen-A3 MHC class I molecules presenting a nine-residue FGF-5 peptide generated by protein splicing. This process, previously described strictly in plants and unicellular organisms, entails post-translational excision of a polypeptide segment followed by ligation of the newly liberated carboxy-terminal and amino-terminal residues. The occurrence of protein splicing in vertebrates has important implications for the complexity of the vertebrate proteome and for the immune recognition of self and foreign peptides.