Interplay between differentially expressed enzymes contributes to light color acclimation in marine <i>Synechococcus</i>.

Abstract

Marine <i>Synechococcus</i>, a globally important group of cyanobacteria, thrives in various light niches in part due to its varied photosynthetic light-harvesting pigments. Many <i>Synechococcus</i> strains use a process known as chromatic acclimation to optimize the ratio of two chromophores, green-light-absorbing phycoerythrobilin (PEB) and blue-light-absorbing phycourobilin (PUB), within their light-harvesting complexes. A full mechanistic understanding of how <i>Synechococcus</i> cells tune their PEB to PUB ratio during chromatic acclimation has not yet been obtained. Here, we show that interplay between two enzymes named MpeY and MpeZ controls differential PEB and PUB covalent attachment to the same cysteine residue. MpeY attaches PEB to the light-harvesting protein MpeA in green light, while MpeZ attaches PUB to MpeA in blue light. We demonstrate that the ratio of <i>mpeY</i> to <i>mpeZ</i> mRNA determines if PEB or PUB is attached. Additionally, strains encoding only MpeY or MpeZ do not acclimate. Examination of strains of <i>Synechococcus</i> isolated from across the globe indicates that the interplay between MpeY and MpeZ uncovered here is a critical feature of chromatic acclimation for marine <i>Synechococcus</i> worldwide.