Phytochromes (PHYs) are photoreceptor proteins first discovered in plants, where they control a variety of photomorphogenesis events. PHYs as photochromic proteins can reversibly switch between two distinct states: a red light (Pr) and a far-red light (Pfr) absorbing form. The discovery of Bacteriophytochromes (BphPs) in nonphotosynthetic bacteria has opened new frontiers in our understanding of the mechanisms by which these natural photoswitches can control single cell development, although the role of BphPs in vivo remains largely unknown. BphPs are dimeric proteins that consist of a photosensory core module (PCM) and an enzymatic domain, often a histidine kinase. The PCM is composed of three domains (PAS, GAF, and PHY). It holds a covalently bound open-chain tetrapyrrole (biliverdin, BV) chromophore. Upon absorption of light, the double bond between BV rings C and D isomerizes and reversibly switches the protein between Pr and Pfr states. We report crystal structures of the wild-type and mutant (His275Thr) forms of the canonical BphP from the nonphotosynthetic myxobacterium Stigmatella aurantiaca (SaBphP2) in the Pr state. Structures were determined at 1.65 Å and 2.2 Å (respectively), the highest resolution of any PCM construct to date. We also report the room temperature wild-type structure of the same protein determined at 2.1 Å at the SPring-8 Angstrom Compact free electron LAser (SACLA), Japan. Our results not only highlight and confirm important amino acids near the chromophore that play a role in Pr-Pfr photoconversion but also describe the signal transduction into the PHY domain which moves across tens of angstroms after the light stimulus.

中文翻译：

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低温和室温下粘细菌光敏色素的高分辨率晶体结构。

光敏色素 (PHY) 是首先在植物中发现的光感受器蛋白，它们控制着多种光形态发生事件。PHY 作为光致变色蛋白可以在两种不同状态之间可逆地切换：红光 (Pr) 和远红光 (Pfr) 吸收形式。尽管 BphP 在体内的作用仍知之甚少，但非光合细菌中细菌光合色素 (BphP) 的发现为我们理解这些天然光开关控制单细胞发育的机制开辟了新的领域。BphP 是二聚体蛋白，由光感核心模块 (PCM) 和酶结构域（通常是组氨酸激酶）组成。PCM 由三个域（PAS、GAF 和 PHY）组成。它含有共价结合的开链四吡咯（胆绿素，BV）发色团。吸收光后，BV 环 C 和 D 之间的双键异构化，并可逆地将蛋白质在 Pr 和 Pfr 状态之间切换。我们报告了来自非光合作用橙色粘杆菌 (SaBphP2) 的 Pr 态典型 BphP 的野生型和突变型 (His275Thr) 晶体结构。结构分别以 1.65 Å 和 2.2 Å 确定，这是迄今为止任何 PCM 结构的最高分辨率。我们还报告了在日本 SPring-8 Angstrom Compact 自由电子激光激光器 (SACLA) 上以 2.1 Å 测定的相同蛋白质的室温野生型结构。我们的结果不仅强调并证实了发色团附近在 Pr-Pfr 光转换中发挥作用的重要氨基酸，而且还描​​述了信号转导到 PHY 域，该信号在光刺激后移动了数十埃。