Adaptation to Blue Light in Marine Requires MpeU, an Enzyme with Similarity to Phycoerythrobilin Lyase Isomerases.

Marine has successfully adapted to environments with different light colors, which likely contributes to this genus being the second most abundant group of microorganisms worldwide. Populations of that grow in deep, blue ocean waters contain large amounts of the blue-light absorbing chromophore phycourobilin (PUB) in their light harvesting complexes (phycobilisomes). Here, we show that all strains adapted to blue light possess a gene called . MpeU is structurally similar to phycobilin lyases, enzymes that ligate chromophores to phycobiliproteins. Interruption of caused a reduction in PUB content, impaired phycobilisome assembly and reduced growth rate more strongly in blue than green light. When was reintroduced in the mutant background, the less phenotype was complemented in terms of PUB content and phycobilisome content. Fluorescence spectra of mutant cells and purified phycobilisomes revealed red-shifted phycoerythrin emission peaks, likely indicating a defect in chromophore ligation to phycoerythrin-I (PE-I) or phycoerythrin-II (PE-II). Our results suggest that MpeU is a lyase-isomerase that attaches a phycoerythrobilin to a PEI or PEII subunit and isomerizes it to PUB. MpeU is therefore an important determinant in adaptation of spp. to capture photons in blue light environments throughout the world's oceans.