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Proceeding of: 2019 IEEE International Symposium on Information Theory (ISIT), 7-12 July 2019, Paris, France ; This paper presents saddlepoint approximations of state-of-the-art converse and achievability bounds for noncoherent, single-antenna, Rayleigh block-fading channels. These approximations can be calculated efficiently and are shown to be accurate for SNR values as small as 0 dB, blocklengths of 168 channel uses or more, and when the channel's coherence interval is not smaller than two. It is demonstrated that the derived approximations recover both the normal approximation and the reliability function of the channel. ; A. Lancho, G. Vázquez-Vilar and T. Koch have received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (714161), the Spanish Ministerio de Economía y Competitividad (RYC-2014-16332, TEC2016-78434-C3-3-R (AEI/FEDER, EU) and IJCI2015-27020), the Spanish Ministerio de Educación, Cultura y Deporte (FPU014/01274), and the Comunidad de Madrid (S2103/ICE-2845). G. Durisi and J. Östman have been supported by the Swedish Research Council under Grants 2016-03293 and 2014-6066.

This paper studies the random-coding exponent of joint source-channel coding for a scheme where source messages are assigned to disjoint subsets (referred to as classes), and codewords are independently generated according to a distribution that depends on the class index of the source message. For discrete memoryless systems, two optimally chosen classes and product distributions are found to be sufficient to attain the sphere-packing exponent in those cases where it is tight. ; This work has been funded in part by the European Research Council (ERC) under grant agreement 259663; by the European Union under the 7th Framework Programme, grants FP7-PEOPLE-2009-IEF no. 252663, FP7-PEOPLE-2011-CIG no. 303633, FP7-PEOPLE-2012-CIG no. 333680, FP7- PEOPLE-2013-IEF no. 329837; and by the Spanish Ministry of Economy and Competitiveness under grants CSD2008-00010, TEC2009-14504-C02- 01, TEC2012-38800-C03-01, TEC2012-38800-C03-03 and RYC-2011-08150. A. Tauste Campo acknowledges funding from an EPSRC (Engineering and Physical Sciences Research Council, UK) Doctoral Prize Award.

This paper studies the random-coding exponent of joint source-channel coding for a scheme where source messages are assigned to disjoint subsets (referred to as classes), and codewords are independently generated according to a distribution that depends on the class index of the source message. For discrete memoryless systems, two optimally chosen classes and product distributions are found to be sufficient to attain the sphere-packing exponent in those cases where it is tight. ; This work was supported in part by the European Research Council under Grant 259663, in part by the European Union under the 7th Framework Programme under Grant FP7-PEOPLE-2009-IEF 252663, Grant FP7-PEOPLE-2011-CIG 303633, Grant FP7-PEOPLE-2012-CIG 333680, and Grant FP7-PEOPLE-2013-IEF 329837, and in part by the Spanish Ministry of Economy and Competitiveness under Grant CSD2008-00010, Grant TEC2009-14504-C02-01, Grant TEC2012-38800-C03-01, Grant TEC2012-38800-C03-03, and Grant RYC-2011-08150.

Funding Information: We thank Laurenz Heidrich for help with statistical analyses. This work was supported by grant Da 351/8‐1 (to CD) from the Deutsche Forschungsgemeinschaft and Fundação para a Ciência e Tecnologia (Portugal) (grant PTDC/BIA‐BQM/29118 and R&D units MOSTMICRO‐ITQB (UIDB/04612/2020 and UIDP/04612/2020), and European Union's Horizon 2020 research and innovation program (grant agreement No 810856). Open access funding enabled and organized by Projekt DEAL. Publisher Copyright: © 2020 The Authors. The FEBS Journal published by John Wiley & Sons Ltd on behalf of Federation of European Biochemical Societies Copyright: Copyright 2021 Elsevier B.V., All rights reserved. ; Many bacteria and archaea employ a novel pathway of sulfur oxidation involving an enzyme complex that is related to the heterodisulfide reductase (Hdr or HdrABC) of methanogens. As a first step in the biochemical characterization of Hdr-like proteins from sulfur oxidizers (sHdr), we structurally analyzed the recombinant sHdrA protein from the Alphaproteobacterium Hyphomicrobium denitrificans at 1.4 Å resolution. The sHdrA core structure is similar to that of methanogenic HdrA (mHdrA) which binds the electron-bifurcating flavin adenine dinucleotide (FAD), the heart of the HdrABC-[NiFe]-hydrogenase catalyzed reaction. Each sHdrA homodimer carries two FADs and two [4Fe–4S] clusters being linked by electron conductivity. Redox titrations monitored by electron paramagnetic resonance and visible spectroscopy revealed a redox potential between −203 and −188 mV for the [4Fe–4S] center. The potentials for the FADH•/FADH− and FAD/FADH• pairs reside between −174 and −156 mV and between −81 and −19 mV, respectively. The resulting stable semiquinone FADH• species already detectable in the visible and electron paramagnetic resonance spectra of the as-isolated state of sHdrA is incompatible with basic principles of flavin-based electron bifurcation such that the sHdr complex does not apply this new mode of energy coupling. The inverted one-electron FAD redox potentials of sHdr and mHdr are clearly reflected in the different FAD-polypeptide interactions. According to this finding and the assumption that the sHdr complex forms an asymmetric HdrAA′B1C1B2C2 hexamer, we tentatively propose a mechanism that links protein-bound sulfane oxidation to sulfite on HdrB1 with NAD+ reduction via lipoamide disulfide reduction on HdrB2. The FAD of HdrA thereby serves as an electron storage unit. Database: Structural data are available in PDB database under the accession number 6TJR. ; published