Black hole physics. Black hole lightning due to particle acceleration at subhorizon scales.

Supermassive black holes with masses of millions to billions of solar masses are commonly found in the centers of galaxies. Astronomers seek to image jet formation using radio interferometry but still suffer from insufficient angular resolution. An alternative method to resolve small structures is to measure the time variability of their emission. Here we report on gamma-ray observations of the radio galaxy IC 310 obtained with the MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov) telescopes, revealing variability with doubling time scales faster than 4.8 min. Causality constrains the size of the emission region to be smaller than 20% of the gravitational radius of its central black hole. We suggest that the emission is associated with pulsar-like particle acceleration by the electric field across a magnetospheric gap at the base of the radio jet.

Observation of pulsed gamma-rays above 25 GeV from the Crab pulsar with MAGIC.

One fundamental question about pulsars concerns the mechanism of their pulsed electromagnetic emission. Measuring the high-end region of a pulsar's spectrum would shed light on this question. By developing a new electronic trigger, we lowered the threshold of the Major Atmospheric gamma-ray Imaging Cherenkov (MAGIC) telescope to 25 giga-electron volts. In this configuration, we detected pulsed gamma-rays from the Crab pulsar that were greater than 25 giga-electron volts, revealing a relatively high cutoff energy in the phase-averaged spectrum. This indicates that the emission occurs far out in the magnetosphere, hence excluding the polar-cap scenario as a possible explanation of our measurement. The high cutoff energy also challenges the slot-gap scenario.

Very-high-energy gamma rays from a distant quasar: how transparent is the universe?

The atmospheric Cherenkov gamma-ray telescope MAGIC, designed for a low-energy threshold, has detected very-high-energy gamma rays from a giant flare of the distant Quasi-Stellar Radio Source (in short: radio quasar) 3C 279, at a distance of more than 5 billion light-years (a redshift of 0.536). No quasar has been observed previously in very-high-energy gamma radiation, and this is also the most distant object detected emitting gamma rays above 50 gigaelectron volts. Because high-energy gamma rays may be stopped by interacting with the diffuse background light in the universe, the observations by MAGIC imply a low amount for such light, consistent with that known from galaxy counts.

Variable very-high-energy gamma-ray emission from the microquasar LS I +61 303.

Microquasars are binary star systems with relativistic radio-emitting jets. They are potential sources of cosmic rays and can be used to elucidate the physics of relativistic jets. We report the detection of variable gamma-ray emission above 100 gigaelectron volts from the microquasar LS I 61 + 303. Six orbital cycles were recorded. Several detections occur at a similar orbital phase, which suggests that the emission is periodic. The strongest gamma-ray emission is not observed when the two stars are closest to one another, implying a strong orbital modulation of the emission or absorption processes.

An in vitro reducing system for the enzymic conversion of cobalamin to adenosylcobalamin.

Homogeneous ferredoxin (flavodoxin):NADP(+) reductase and flavodoxin A proteins served as electron donors for the reduction of co(III)rrinoids to co(I)rrinoids in vitro. The resulting co(I)rrinoids served as substrates for the ATP:co(I)rrinoid adenosyltransferase (CobA) enzyme of Salmonella enterica serovar Typhimurium LT2 and were converted to their respective adenosylated derivatives. The reaction products were isolated by reverse phase high performance liquid chromatography, and their identities were confirmed by UV-visible spectroscopy, mass spectrometry, and in vivo biological activity assays. Adenosylcobalamin generated by this system supported the activity of 1,2-propanediol dehydratase as effectively as authentic adenosylcobalamin. This is the first report of a protein system that can be coupled to the adenosyltransferase CobA enzyme for the conversion of co(III)rrinoids to their adenosylated derivatives.

Three-dimensional structure of ATP:corrinoid adenosyltransferase from Salmonella typhimurium in its free state, complexed with MgATP, or complexed with hydroxycobalamin and MgATP.

In Salmonella typhimurium, formation of the cobalt-carbon bond in the biosynthetic pathway for adenosylcobalamin is catalyzed by the product of the cobA gene which encodes a protein of 196 amino acid residues. This enzyme is an ATP:co(I)rrinoid adenosyltransferase which transfers an adenosyl moiety from MgATP to a broad range of co(I)rrinoid substrates that are believed to include cobinamide, its precursor cobyric acid and probably others as yet unidentified, and hydroxocobalamin. Three X-ray structures of CobA are reported here: its substrate-free form, a complex of CobA with MgATP, and a ternary complex of CobA with MgATP and hydroxycobalamin to 2.1, 1.8, and 2.1 A resolution, respectively. These structures show that the enzyme is a homodimer. In the apo structure, the polypeptide chain extends from Arg(28) to Lys(181) and consists of an alpha/beta structure built from a six-stranded parallel beta-sheet with strand order 324516. The topology of this fold is very similar to that seen in RecA protein, helicase domain, F(1)ATPase, and adenosylcobinamide kinase/adenosylcobinamide guanylyltransferase where a P-loop is located at the end of the first strand. Strikingly, the nucleotide in the MgATP.CobA complex binds to the P-loop of CobA in the opposite orientation compared to all the other nucleotide hydrolases. That is, the gamma-phosphate binds at the location normally occupied by the alpha-phosphate. The unusual orientation of the nucleotide arises because this enzyme transfers an adenosyl group rather than the gamma-phosphate. In the ternary complex, the binding site for hydroxycobalamin is located in a shallow bowl-shaped depression at the C-terminal end of the beta-sheet of one subunit; however, the active site is capped by the N-terminal helix from the symmetry-related subunit that now extends from Gln(7) to Ala(24). The lower ligand of cobalamin is well-ordered and interacts mostly with the N-terminal helix of the symmetry-related subunit. Interestingly, there are few interactions between the protein and the polar side chains of the corrin ring which accounts for the broad specificity of this enzyme. The corrin ring is oriented such that the cobalt atom is located approximately 6.1 A from C5' of the ribose and is beyond the range of nucleophilic attack. This suggests that a conformational change occurs in the ternary complex when Co(III) is reduced to Co(I).

Reduction of Cob(III)alamin to Cob(II)alamin in Salmonella enterica serovar typhimurium LT2.

Reduction of the cobalt ion of cobalamin from the Co(III) to the Co(I) oxidation state is essential for the synthesis of adenosylcobalamin, the coenzymic form of this cofactor. A cob(II)alamin reductase activity in Salmonella enterica serovar Typhimurium LT2 was isolated to homogeneity. N-terminal analysis of the homogeneous protein identified NAD(P)H:flavin oxidoreductase (Fre) (EC 1.6.8.1) as the enzyme responsible for this activity. The fre gene was cloned, and the overexpressed protein, with a histidine tag at its N terminus, was purified to homogeneity by nickel affinity chromatography. His-tagged Fre reduced flavins (flavin mononucleotide [FMN] and flavin adenine dinucleotide [FAD]) and cob(III)alamin to cob(II)alamin very efficiently. Photochemically reduced FMN substituted for Fre in the reduction of cob(III)alamin to cob(II)alamin, indicating that the observed cobalamin reduction activity was not Fre dependent but FMNH(2) dependent. Enzyme-independent reduction of cob(III)alamin to cob(II)alamin by FMNH(2) occurred at a rate too fast to be measured. The thermodynamically unfavorable reduction of cob(II)alamin to cob(I)alamin was detectable by alkylation of the cob(I)alamin nucleophile with iodoacetate. Detection of the product, caboxymethylcob(III)alamin, depended on the presence of FMNH(2) in the reaction mixture. FMNH(2) failed to substitute for potassium borohydride in in vitro assays for corrinoid adenosylation catalyzed by the ATP:co(I)rrinoid adenosyltransferase (CobA) enzyme, even under conditions where Fre and NADH were present in the reaction mixture to ensure that FMN was always reduced. These results were interpreted to mean that Fre was not responsible for the generation of cob(I)alamin in vivo. Consistent with this idea, a fre mutant displayed wild-type cobalamin biosynthetic phenotypes. It is proposed that S. enterica serovar Typhimurium LT2 may not have a cob(III)alamin reductase enzyme and that, in vivo, nonadenosylated cobalamin and other corrinoids are maintained as co(II)rrinoids by reduced flavin nucleotides generated by Fre and other flavin oxidoreductases.