Coenzyme B12-dependent and independent photoregulation of carotenogenesis across Myxococcales

Abstract

Light-induced carotenogenesis in Myxococcus xan- thus is controlled by the B12-based CarH repressor and photoreceptor, and by a separate intricate path- way involving singlet oxygen, the B12-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 ana- lyses 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 B12- mediated photoregulated carotenogenesis in repre- sentative 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 analy- sis in vitro) is a dimer that combines direct CarH-like B12-based photoregulation with CarA-like DNA binding and inhibition by an antirepressor. This study pro- vides new molecular insights into B12-dependent pho- toreceptors. It further establishes the B12-dependent pathway for photoregulated carotenogenesis as broadly prevalent across myxobacteria and its evolu- tion, exclusively in one suborder, into a parallel com- plex B12-independent circuit.