title: Adaptation to Blue Light in MarineSynechococcusRequires MpeU, an Enzyme with Similarity to Phycoerythrobilin Lyase Isomerases.
abstract: MarineSynechococcushas successfully adapted to environments with different light colors, which likely contributes to this genus being the second most abundant group of microorganisms worldwide. Populations ofSynechococcusthat 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 allSynechococcusstrains adapted to blue light possess a gene calledmpeU. MpeU is structurally similar to phycobilin lyases, enzymes that ligate chromophores to phycobiliproteins. Interruption ofmpeUcaused a reduction in PUB content, impaired phycobilisome assembly and reduced growth rate more strongly in blue than green light. WhenmpeUwas reintroduced in thempeUmutant background, thempeU-less phenotype was complemented in terms of PUB content and phycobilisome content. Fluorescence spectra ofmpeUmutant 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 ofSynechococcusspp. to capture photons in blue light environments throughout the world's oceans.
authors.affiliation: Department of Biology, Indiana University, BloomingtonIN, USA; Department of Botany, Faculty of Science, University of FayoumFayoum, Egypt.
authors.forename: Rania M
authors.initials: RM
authors.lastname: Mahmoud
authors.affiliation: Department of Biology, Indiana University, Bloomington IN, USA.
authors.forename: Joseph E
authors.initials: JE
authors.lastname: Sanfilippo
authors.affiliation: Department of Biological Sciences, University of New Orleans, New OrleansLA, USA; Department of Chemistry, University of New Orleans, New OrleansLA, USA.
authors.forename: Adam A
authors.initials: AA
authors.lastname: Nguyen
authors.affiliation: Department of Biology, Indiana University, Bloomington IN, USA.
authors.forename: Johann A
authors.initials: JA
authors.lastname: Strnat
authors.affiliation: CNRS, Sorbonne Universités, Université Pierre et Marie Curie University Paris 06, UMR 7144 Roscoff, France.
authors.forename: Frédéric
authors.initials: F
authors.lastname: Partensky
authors.affiliation: CNRS, Sorbonne Universités, Université Pierre et Marie Curie University Paris 06, UMR 7144 Roscoff, France.
authors.forename: Laurence
authors.initials: L
authors.lastname: Garczarek
authors.affiliation: Department of Botany, Faculty of Science, University of Fayoum Fayoum, Egypt.
authors.forename: Nabil
authors.initials: N
authors.lastname: Abo El Kassem
authors.affiliation: Department of Biology, Indiana University, BloomingtonIN, USA; Indiana Molecular Biology Institute, Indiana University, BloomingtonIN, USA.
authors.forename: David M
authors.initials: DM
authors.lastname: Kehoe
authors.affiliation: Department of Biological Sciences, University of New Orleans, New OrleansLA, USA; Department of Chemistry, University of New Orleans, New OrleansLA, USA.
authors.forename: Wendy M
authors.initials: WM
authors.lastname: Schluchter
keywords: blue light; lyase isomerase; marine Synechococcus; marine cyanobacteria; phycobilin; phycobilisome; phycoerythrin; phycourobilin