Cyanobacteriochromes (CBCRs) are phytochrome-related photoreceptor proteins in cyanobacteria and cover a wide spectral range from ultraviolet to far-red. A single GAF domain that they contain can bind bilin(s) autocatalytically via heterologous recombination and then fluoresce, with potential applications as biomarkers and biosensors. Here, we report that a novel red/green CBCR GAF domain, SPI1085g2, from Spirulina subsalsa covalently binds both phycocyanobilin (PCB) and phycoerythrobilin (PEB). The PCB-binding GAF domain exhibited canonical red/green photoconversion with weak fluorescence emission. However, the PEB-binding GAF domain, SPI1085g2-PEB, exhibited an intense orange fluorescence (λ_{abs. max} = 520 nm, λ_{fluor. max}= 555 nm), with a fluorescence quantum yield close to 1.0. The fluorescence of SPI1085g2-PEB was selectively and instantaneously quenched by copper ions in a concentration-dependent manner and exhibited reversibility upon treatment with the metal chelator EDTA. This study identified a novel PEB-binding cyanobacteriochrome-based fluorescent protein with the highest quantum yield reported to date and indicated its potential as a biosensor for the rapid detection of copper ions.

中文翻译：

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来自螺旋藻的新型蓝藻色素 GAF 结构域的色基化及其对铜离子的反应。

蓝藻色素 (CBCR) 是蓝藻中与光敏色素相关的光感受器蛋白，涵盖从紫外线到远红光的宽光谱范围。它们包含的单个 GAF 结构域可以通过异源重组自催化结合胆汁素，然后发出荧光，具有作为生物标志物和生物传感器的潜在应用。在这里，我们报告了一种来自Spirulina subsalsa 的新型红/绿 CBCR GAF 结构域 SPI1085g2共价结合藻蓝蛋白 (PCB) 和藻红蛋白 (PEB)。PCB 结合的 GAF 结构域表现出典型的红/绿光转换，荧光发射较弱。然而，结合 PEB 的 GAF 结构域 SPI1085g2-PEB 表现出强烈的橙色荧光（λ _{abs. max} = 520 nm，λ _{fluor. max}= 555 nm)，荧光量子产率接近 1.0。SPI1085g2-PEB 的荧光以浓度依赖性方式被铜离子选择性瞬时淬灭，并在用金属螯合剂 EDTA 处理后表现出可逆性。本研究确定了一种新型 PEB 结合蓝藻色素基荧光蛋白，该蛋白具有迄今为止报道的最高量子产率，并表明其作为快速检测铜离子的生物传感器的潜力。