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Cover Illustration: WrbA of E. coli is the founding member of a large conserved family of novel flavodoxin-like proteins distributed from bacteria through fungi to higher plants. Aside from the signature motif for FMN binding, WrbA sequences present several distinctions from the monomeric flavodoxins. X-ray crystallography was used to examine the structure of WrbA with oxidized FMN bound. Here, tetrameric WrbA is represented with each subunit a unique ribbon color showing one FMN per subunit in skeletal model facing the outside of the assembly. The crystal structure reveals a relationship to dimeric eukaryotic FAD-dependent DT-diaphorases, which have been thought to function in protection of cells from xenobiotic electrophiles. Like the diaphorases, WrbAs have two-electron NAD(P)H:quinone oxidoreductase activity, but no definitive physiological role has been established for either protein. Comparative structural analysis reveals how WrbA is adapted to function as an obligate tetramer despite having no dedicated multimerization domain. The poles of the WrbA tetramer present hydrophobic regions leading to the active sites, suggesting that the protein may interact at least transiently with membranes. (See Carey et al., pp. 2301-2305.)