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Filamentous enzymes have been found in all domains of life, but the advantage of filamentation is often elusive1. Some anaerobic, autotrophic bacteria have an unusual filamentous enzyme for CO2 fixation—hydrogen-dependent CO2 reductase (HDCR)2,3—which directly converts H2 and CO2 into formic acid. HDCR reduces CO2 with a higher activity than any other known biological or chemical catalyst4,5, and it has therefore gained considerable interest in two areas of global relevance: hydrogen storage and combating climate change by capturing atmospheric CO2. However, the mechanistic basis of the high catalytic turnover rate of HDCR has remained unknown. Here we use cryo-electron microscopy to reveal the structure of a short HDCR filament from the acetogenic bacterium Thermoanaerobacter kivui. The minimum repeating unit is a hexamer that consists of a formate dehydrogenase (FdhF) and two hydrogenases (HydA2) bound around a central core of hydrogenase Fe-S subunits, one HycB3 and two HycB4. These small bacterial polyferredoxin-like proteins oligomerize through their C-terminal helices to form the backbone of the filament. By combining structure-directed mutagenesis with enzymatic analysis, we show that filamentation and rapid electron transfer through the filament enhance the activity of HDCR. To investigate the structure of HDCR in situ, we imaged T. kivui cells with cryo-electron tomography and found that HDCR filaments bundle into large ring-shaped superstructures attached to the plasma membrane. This supramolecular organization may further enhance the stability and connectivity of HDCR to form a specialized metabolic subcompartment within the cell.
The basic problem of primary audio and video research materials is clearly shown by the survey: A great and important part of the entire heritage is still outside archival custody in the narrower sense, scattered over many institutions in fairy small collections, and even in private hands. reservation following generally accepted standards can only be carried out effectively if collections represent critical mass. Specialised audiovisual archives will solve their problems, as they will sooner or later succeed in getting appropriate funding to achieve their aims. A very encouraging example is the case of the Netherlands. The larger audiovisual research archives will also manage, more or less autonomously, the transfer of contents in time. For a considerable part of the research collections, however, the concept of cooperative models and competence centres is the only viable model to successfullly safeguard their holdings. Their organisation and funding is a considerable challenge for the scientific community. TAPE has significantly raised awareness of the fact that, unless action is swiftly taken, the loss of audiovisual materials is inevitable. TAPE’s international and regional workshops were generally overbooked. While TAPE was already underway, several other projects for the promotion of archives have received grants from organisations other than the European Commission, inter alia support for the St. Petersburg Phonogram Archive, and the Folklore Archive in Tirana, obviously as a result of a better understanding of the need for audiovisual preservation. When the TAPE project started its partners assumed that cooperative projects would fail because of the notorious distrust of researchers, specifically in the post-communist countries. One of the most encouraging surprises was to learn that, at least in the most recent survey, it became apparent that this social obstacle is fading out. TAPE may have contributed to this important development.