Distant cousin helps spot animal enzyme: In addition to forming the membranes that enclose cells, lipids are important signaling molecules. Plasmalogens, which contain a vinyl ether linkage, are an abundant group of lipids in animals. How these lipids are synthesized from precursors with an alkyl ether linkage has been a mystery. Gallego-García et al. found an enzyme, CarF, in the social bacterium Myxococcus xanthus that produces plasmalogens used in a signaling pathway for singlet oxygen, a marker of photooxidative stress. They then showed that the animal homolog could catalyze the final step in plasmalogen synthesis in bacterial and human cells, thus resolving a source for plasmalogens in animals. Science , this issue p. 128

Myxobacteria are Gram-negative δ-proteobacteria found predominantly in terrestrial habitats and often brightly colored due to the biosynthesis of carotenoids. Carotenoids are lipophilic isoprenoid pigments that protect cells from damage and death by quenching highly reactive and toxic oxidative species, like singlet oxygen, generated upon growth under light. The model myxobacterium Myxococcus xanthus turns from yellow in the dark to red upon exposure to light because of the photoinduction of carotenoid biosynthesis. How light is sensed and transduced to bring about regulated carotenogenesis in order to combat photooxidative stress has been extensively investigated in M. xanthus using genetic, biochemical and high-resolution structural methods. These studies have unearthed new paradigms in bacterial light sensing, signal transduction and gene regulation, and have led to the discovery of prototypical members of widely distributed protein families with novel functions. Major advances have been made over the last decade in elucidating the molecular mechanisms underlying the light-dependent signaling and regulation of the transcriptional response leading to carotenogenesis in M. xanthus. This review aims to provide an up-to-date overview of these findings and their significance.

Newly discovered bacterial photoreceptors called CarH sense light by using 5′-deoxyadenosylcobalamin (AdoCbl). They repress their own expression and that of genes for carotenoid synthesis by binding in the dark to operator DNA as AdoCbl-bound tetramers, whose light-induced disassembly relieves repression. High-resolution structures of Thermus thermophilus CarH_Tt have provided snapshots of the dark and light states and have revealed a unique DNA-binding mode whereby only three out of four DNA binding domains contact an operator comprising three tandem direct repeats. To gain further insights into CarH photoreceptors and employing biochemical, spectroscopic, mutational and computational analyses, here we investigated CarH_Bm from Bacillus megaterium. We found that apoCarH_Bm, unlike monomeric apoCarH_Tt, is an oligomeric molten globule that forms DNA-binding tetramers in the dark only upon AdoCbl binding, which requires a conserved W-x₉-EH motif. Light relieved DNA binding by disrupting CarH_Bm tetramers to dimers, rather than to monomers as with CarH_Tt. CarH_Bm operators resembled that of CarH_Tt, but were larger by one repeat and overlapped with the -35 or -10 promoter elements. This design persisted in a six-repeat, multipartite operator we discovered upstream of a gene encoding an Spx global redox-response regulator whose photoregulated expression links photooxidative and general redox responses in B. megaterium. Interestingly, CarH_Bm recognized the smaller CarH_Tt operator, revealing an adaptability possibly related to the linker bridging the DNA- and AdoCbl-binding domains. Our findings highlight a remarkable plasticity in the mode of action of B₁₂-based CarH photoreceptors, important for their biological functions and development as optogenetic tools.

Summary: Light‐induced carotenogenesis in Myxococcus xanthus is controlled by the B ₁₂ ‐based CarH repressor and photoreceptor, and by a separate intricate pathway involving singlet oxygen, the B ₁₂ ‐independent CarH paralogue CarA and various other proteins, some eukaryotic‐like. Whether other myxobacteria conserve these pathways and undergo photoregulated carotenogenesis is unknown. Here, comparative analyses across 27 Myxococcales genomes identified carotenogenic genes, albeit arranged differently, with carH often in their genomic vicinity, in all three Myxococcales suborders. However, CarA and its associated factors were found exclusively in suborder Cystobacterineae, with carA ‐ carH invariably in tandem in a syntenic carotenogenic operon, except for Cystobacter/Melittangium , which lack CarA but retain all other factors. We experimentally show B ₁₂ ‐mediated photoregulated carotenogenesis in representative myxobacteria, and a remarkably plastic CarH operator design and DNA binding across Myxococcales. Unlike the two characterized CarH from other phyla, which are tetrameric, Cystobacter CarH (the first myxobacterial homologue amenable to analysis in vitro ) is a dimer that combines direct CarH‐like B ₁₂ ‐based photoregulation with CarA‐like DNA binding and inhibition by an antirepressor. This study provides new molecular insights into B ₁₂ ‐dependent photoreceptors. It further establishes the B ₁₂ ‐dependent pathway for photoregulated carotenogenesis as broadly prevalent across myxobacteria and its evolution, exclusively in one suborder, into a parallel complex B ₁₂ ‐independent circuit.