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Near-infrared imaging in fission yeast by genetically encoded biosynthesis of phycocyanobilin.

Near-infrared fluorescent protein (iRFP) is a bright and stable fluorescent protein with near-infrared excitation and emission maxima. Unlike the other conventional fluorescent proteins, iRFP requires biliverdin (BV) as a chromophore. Here, we report that phycocyanobilin (PCB) functions as a brighter chromophore for iRFP than BV, and biosynthesis of PCB allows live-cell imaging with iRFP in the fission yeast Schizosaccharomyces pombe. We initially found that fission yeast cells did not produce BV, and therefore did not show any iRFP fluorescence. The brightness of iRFP-PCB was higher than that of iRFP-BV in vitro and in fission yeast. We introduced SynPCB, a PCB biosynthesis system, into fission yeast, resulting in the brightest iRFP fluorescence. To make iRFP readily available in fission yeast, we developed an endogenous gene tagging system with iRFP and all-in-one integration plasmids carrying the iRFP-fused marker proteins together with SynPCB. These tools not only enable the easy use of the multiplexed live-cell imaging in fission yeast with a broader color palette, but also open the door to new opportunities for near-infrared fluorescence imaging in a wider range of living organisms.