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The idea that gas may be present in clusters of galaxies originated with the missing mass problem—the observation that galaxies in clusters were moving so fast that the inferred galaxy masses (using conventional mass to light ratios for the galaxies) were not enough to hold them together caused astronomers to speculate that the “missing mass” was in the form of gas. Subsequent observations in the 21 cm and La lines as well as radio, optical and x-ray continuum observations have effectively ruled out this possibility in a large number of clusters. However, there is evidence for the presence of gas in smaller quantities in many clusters.

Many Gram-negative pathogenic bacteria use a complex macromolecular machine, known as the type 3 secretion system (T3SS), to transfer virulence proteins into host cells. The T3SS is composed of a cytoplasmic bulb, a basal body spanning the inner and outer bacterial membranes, and an extracellular needle. Secretion is regulated by both cytoplasmic and inner membrane proteins that must respond to specific signals in order to ensure that virulence proteins are not secreted before contact with a eukaryotic cell. This negative regulation is mediated, in part, by a family of proteins that are thought to physically block the entrance to the secretion apparatus until an appropriate signal is received following host cell contact. Despite weak sequence homology between proteins of this family, the crystal structures of Shigella flexneri MxiC we present here confirm the conservation of domain topology with the homologue from Yersinia sp. Interestingly, comparison of the Shigella and Yersinia structures reveals a significant structural change that results in substantial domain re-arrangement and opening of one face of the molecule. The conservation of a negatively charged patch on this face suggests it may have a role in binding other components of the T3SS.

A summary is given of the magnetospheric physics of accreting magnetized neutron stars. The role of hydromagnetic instabilities is described, with particular attention to the formation of polar caps in spherical and disk on, and to the origin of spin down torques in a centrigugally driven stellar wind, when the accretion is from a disk. The most elementary consequences for formation of the emergent spectrum are also discussed.

Preliminary results are presented from a two‐dimensional radiation gas dynamics study of polar cap accretion. In particular, we find that photon advection leads to fan beam formation when the accretion mass flux is independent of distance from the magnetic axis and that collisional excitation of cyclotron line photons is at least 10 times more important than bremsstrahlung as a source of photons. We also find evidence for the onset of a new form of overstable convection.

Complement Factor H has recently come to the fore with variant forms implicated in a range of serious disease states. This review aims to bring together recent data concerning the structure and biological activity of this molecule to highlight the way in which a molecular understanding of function may open novel therapeutic possibilities. In particular we examine the evidence for and against the hypothesis that sequence variations in factor H may predispose to disease if they perturb its ability to recognise and respond appropriately to polyanionic carbohydrates on host surfaces that require protection from complement-mediated damage.

Antigenic variation in a major discontinuous site (site D) of foot-and-mouth disease virus (FMDV) of serotype C has been evaluated with neutralizing monoclonal antibodies. Isolates representing the major evolutionary sublines previously defined for serotype C were compared. Extensive variation, comparable to that of continuous epitopes within the hypervariable immunodominant site A (the VP1 G-H loop), was found. The amino acid sequences of the complete capsids of three antigenically highly divergent FMDVs (C1 Haute Loire-Fr/69, C5 Argentina/69, and C3 Argentina/85) have been determined and compared with the corresponding sequences previously determined for seven additional type C viruses. Differences in antigenicity are due to a very limited number of substitutions of surface amino acids accessible to antibodies and located within antigenic sites previously identified on FMDV. A significant number of residues at these positions were also replaced in monoclonal antibody escape mutants. Depending on the variants compared, replacements within site A or at site D, or at both sites, contributed significantly to their antigenic differences. Examples of divergence mediated by a few amino acid replacements were found among FMDVs of Europe and South America. The results suggest that within a serotype of FMDV, antigenically highly divergent viruses can arise in the field by very limited sequence variation at exposed key residues of each of several antigenic sites.

We report the detection of soft (0.2-2.5 keV) X-ray emission from several known cluster X-ray sources using the low energy detectors of the HEAO 1 A-2 experiment. Soft X-ray emission was observed from five clusters - the Centaurus cluster, Abell 2147, SC 1329-314, Abell 2319, and Abell 133. Spectral parameters estimated from the soft X-ray fluxes are inconsistent with those reported at higher energies for the Centaurus cluster, Abell 2147, and SC 1329-314, indicating the presence of more than one spectral component in these clusters. No evidence for more than one component was found for either Abell 2319 or Abell 133. The temperature of Abell 133 is constrained to be less than 2 x 10 to the 7th K, making it the coolest X-ray cluster yet detected.

Data are presented from a HEAO 1 scanning observation of the Virgo cluster in December 1977. The L-shell iron lines at 1.1 keV have been observed in the spectrum of this source. The iron abundance derived is consistent with previous determinations from the 6.7-keV line. The HEAO data also indicate that the source is extended with FWHM of about 1 deg of arc.

Assuming that the accreting material originates in a wind or envelope and therefore carried no net angular momentum, the Kevin-Helmholtz instability is applied to the transport of accreting plasma across the magnetopause of a rotating and magnetized neutron star for the case in which this surface is interchange-stable. Magnetopause location is determined, and the blackbody emission temperature is estimated. A possible period-dependence is noted in the X-ray temperature data which is in approximate agreement with the present model calculations. The magnetopause is found to be insufficiently stable to support an optically thick sheath of plasma surrounding the source, so that neither shrouding of the X-ray source nor significant reprocessing of the radiation field is possible in the model presented. It is concluded that while the flow structure at the magnetopause can control accretion column geometry, the emerging pulse profile and spectra are only indirectly altered, through a dynamical influence on emission region geometry.

X-ray observations are reported for a sample of distant clusters for which the percentage of apparent cluster members that are blue has been determined optically. The X-ray properties of the clusters are not simply related to the optical properties. The X-ray observations do not support models in which blue galaxies change color as a result of ram-pressure stripping, or in which star formation is triggered by interaction with an intracluster medium. The data lend some support to the idea that blue galaxies in distant clusters are being observed immediately after the removal of galactic halos, and before the interstellar medium is depleted (Larson et al., 1980).

We study the decay of primordial random motions of galaxies and galaxy clusters in an expanding universe by solving a kinetic equation for the relaxation of differential energy spectra N(E, t). Systematic dissipative energy losses are included, involving gravitational drag by, and accretion of, intergalactic matter, as well as the effect of collisions with other systems. Formal and numerical solutions are described for two distinct modes of galaxy formation in a turbulent medium, corresponding to formation at a distinct epoch and to continuous formation of galaxies. We show that any primordial random motions of galaxies at the present epoch can amount to at most a few km/sec, and that collisions at early epochs can lead to the acquisition of significant amounts of primordial angular momentum.

Analysis of the X-ray data for the extended sources in Coma, Perseus, and Virgo in terms of an isothermal gas-sphere model emitting thermal-bremsstrahlung radiation. It is found that the gas distribution is somewhat less centrally condensed than the galaxies, and that the gas mass is generally a small fraction of the binding mass of each cluster. In the case of the Coma cluster, approximately one-eighth of the binding mass (with an uncertainty of about 50%) may be in the form of hot gas at 100,000,000 K if H sub zero = 50 km/sec/Mpc.