Cyanobacteriochromes are small, panchromatic photoreceptors in the phytochrome superfamily that regulate diverse light-mediated adaptive processes in cyanobacteria. The molecular basis of far-red (FR) light perception by cyanobacteriochromes is currently unknown. Here we report the crystal structure of a far-red-sensing cyanobacteriochrome from Anabaena cylindrica PCC 7122, which exhibits a reversible far-red/orange photocycle. The 2.7 A structure of its FR-absorbing dark state, determined by room temperature serial crystallography and cryo-crystallography, reveals an all-Z,syn configuration of its bound linear tetrapyrrole (bilin) chromophore that is less extended than the bilin chromophores of all known phytochromes. Based on structural comparisons with other bilin-binding proteins and extensive spectral analyses on mutants, we identify key protein- chromophore interactions that enable far-red sensing in bilin-binding proteins. We propose that FR-CBCRs employ two distinct tuning mechanisms, which work together to produce a large batho-chromatic shift. Findings of this work have important implications for development and improvement of photoproteins with far-red absorption and fluorescence.

中文翻译：

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蓝细菌色素远红外传感的分子基础

蓝细菌色素是植物色素超家族中的小型全色感光体，可调节蓝细菌中各种光介导的适应性过程。蓝细菌色素感知远红光的分子基础目前未知。在这里，我们报告了来自鱼腥藻PCC 7122的深红色感应蓝细菌色素的晶体结构，该色素表现出可逆的远红色/橙色光循环。由室温串联晶体学和冷冻晶体学确定的2.7 A吸收FR的暗态结构揭示了其结合的线性四吡咯（bilin）发色团的全Z，syn构型，其扩展性小于所有Bilin发色团已知的植物色素。基于与其他bilin结合蛋白的结构比较以及对突变体的广泛光谱分析，我们确定了关键的蛋白质-生色团相互作用，从而在结合胆红素的蛋白质中实现了远红外传感。我们建议FR-CBCRs采用两种不同的调谐机制，它们共同作用以产生较大的红移。这项工作的发现对具有远红吸收和荧光的光蛋白的开发和改进具有重要意义。