RESUMO
Active galactic nuclei (AGNs) display many energetic phenomena--broad emission lines, X-rays, relativistic jets, radio lobes--originating from matter falling onto a supermassive black hole. It is widely accepted that orientation effects play a major role in explaining the observational appearance of AGNs. Seen from certain directions, circum-nuclear dust clouds would block our view of the central powerhouse. Indirect evidence suggests that the dust clouds form a parsec-sized torus-shaped distribution. This explanation, however, remains unproved, as even the largest telescopes have not been able to resolve the dust structures. Here we report interferometric mid-infrared observations that spatially resolve these structures in the galaxy NGC 1068. The observations reveal warm (320 K) dust in a structure 2.1 parsec thick and 3.4 parsec in diameter, surrounding a smaller hot structure. As such a configuration of dust clouds would collapse in a time much shorter than the active phase of the AGN, this observation requires a continual input of kinetic energy to the cloud system from a source coexistent with the AGN.
Assuntos
Proteínas do Capsídeo , Reagentes de Ligações Cruzadas/farmacologia , Pirimidinas/farmacologia , RNA/química , RNA/efeitos dos fármacos , Triptofano/análogos & derivados , Tirosina/análogos & derivados , Uridina/análogos & derivados , Sequência de Bases , Bromouracila/análogos & derivados , Capsídeo/química , Elétrons , Eletroforese em Gel de Poliacrilamida , Cinética , Modelos Químicos , Dados de Sequência Molecular , Conformação de Ácido Nucleico , Nucleoproteínas/metabolismo , Ligação Proteica , RNA/efeitos da radiação , Proteínas de Ligação a RNA/química , Fatores de Tempo , Triptofano/farmacologia , Tirosina/farmacologia , Raios Ultravioleta , Uridina/farmacologia , Água/metabolismoRESUMO
Photocross-linking is a useful technique for the partial definition of the nucleic acid-protein interface of nucleoprotein complexes. It can be accomplished by one or two photon excitations of wild-type nucleoprotein complexes or by one photon excitation of nucleoprotein complexes bearing one or more substitutions with photoreactive chromophores. Chromophores that have been incorporated into nucleic acids for this purpose include aryl azides, 5-azidouracil, 8-azidoadenine, 8-azidoguanine, 4-thiouracil, 5-bromouracil, 5-iodouracil, and 5-iodocytosine. The various techniques and chromophores are described and compared, with attention to the photochemical mechanism.
Assuntos
Proteínas Nucleares/química , Ácidos Nucleicos/química , Fotoquímica/métodos , Animais , Reagentes de Ligações Cruzadas , Proteínas de Ligação a DNA/química , Proteínas de Ligação a DNA/metabolismo , Humanos , Proteínas Nucleares/metabolismo , Ácidos Nucleicos/metabolismo , Ligação Proteica , Proteínas de Ligação a RNA/química , Proteínas de Ligação a RNA/metabolismoRESUMO
An analogue of the replicase translational operator of bacteriophage R17, that contains a 5-bromouridine at position -5 (RNA 1), complexes with a dimer of the coat protein and photocrosslinks to the coat protein in high yield upon excitation at 308 nm with a xenon chloride excimer laser. Tryptic digestion of the crosslinked nucleoprotein complex followed by Edman degradation of the tryptic fragment bearing the RNA indicates crosslinking to tyrosine 85 of the coat protein. A control experiment with a Tyr 85 to Ser 85 variant coat protein showed binding but no photocrosslinking at saturating protein concentration. This is consistent with the observation from model compound studies of preferential photocrosslinking of BrU to the electron rich aromatic amino acids tryptophan, tyrosine, and histidine with 308 nm excitation.