Microbial rhodopsins, comprising a protein moiety (rhodopsin apoprotein) bound to the light-absorbing chromophore retinal, function as ion pumps, ion channels, or light sensors. However, recent genomic and metagenomic surveys showed that some rhodopsin-possessing prokaryotes lack the known genes for retinal biosynthesis. Since rhodopsin apoproteins cannot absorb light energy, rhodopsins produced by prokaryotic strains lacking genes for retinal biosynthesis are hypothesized to be non-functional in cells. In the present study, we investigated whether Aurantimicrobium minutum KNCT, which is widely distributed in terrestrial environments and lacks any previously identified retinal biosynthesis genes, possesses functional rhodopsin. We initially measured ion transport activity in cultured cells. A light-induced pH change in a cell suspension of rhodopsin-possessing bacteria was detected in the absence of exogenous retinal. Furthermore, spectroscopic analyses of the cell lysate and HPLC-MS/MS analyses revealed that this strain contained an endogenous retinal. These results confirmed that A. minutum KNCT possesses functional rhodopsin and, hence, produces retinal via an unknown biosynthetic pathway. These results suggest that rhodopsin-possessing prokaryotes lacking known retinal biosynthesis genes also have functional rhodopsins.

中文翻译：

---

缺乏已知的视网膜生物合成基因的细菌构建了功能性视紫红质

微生物视紫红质，包含与光吸收发色团视网膜结合的蛋白质部分（视紫红质载脂蛋白），用作离子泵、离子通道或光传感器。然而，最近的基因组和宏基因组调查表明，一些含有视紫红质的原核生物缺乏已知的视网膜生物合成基因。由于视紫质脱辅基蛋白不能吸收光能，因此假设由缺乏视网膜生物合成基因的原核菌株产生的视紫质在细胞中没有功能。在本研究中，我们调查了广泛分布在陆地环境中且缺乏任何先前确定的视网膜生物合成基因的 Aurantimicrobium minutum KNCT 是否具有功能性视紫质。我们最初测量了培养细胞中的离子转运活性。在没有外源性视黄醛的情况下，检测到含有视紫红质的细菌细胞悬液中光诱导的 pH 值变化。此外，细胞裂解物的光谱分析和 HPLC-MS/MS 分析表明，该菌株含有内源性视网膜。这些结果证实 A. minutum KNCT 具有功能性视紫质，因此通过未知的生物合成途径产生视黄醛。这些结果表明，缺乏已知视网膜生物合成基因的具有视紫质的原核生物也具有功能性视紫质。通过未知的生物合成途径产生视黄醛。这些结果表明，缺乏已知视网膜生物合成基因的具有视紫质的原核生物也具有功能性视紫质。通过未知的生物合成途径产生视黄醛。这些结果表明，缺乏已知视网膜生物合成基因的具有视紫质的原核生物也具有功能性视紫质。