Bacterium Lacking a Known Gene for Retinal Biosynthesis Constructs Functional Rhodopsins

• Published in: Microbes and Environments Vol. 35; no. 4; p. ME20085
• Main Authors: Nakajima, Yu, Kojima, Keiichi, Kashiyama, Yuichiro, Doi, Satoko, Nakai, Ryosuke, Sudo, Yuki, Kogure, Kazuhiro, Yoshizawa, Susumu
• Format: Journal Article
• Language: English
• Published: Japan Japanese Society of Microbial Ecology / Japanese Society of Soil Microbiology / Taiwan Society of Microbial Ecology / Japanese Society of Plant Microbe Interactions / Japanese Society for Extremophiles 2020
• Subjects: Actinobacteria - chemistry; Actinobacteria - genetics; Actinobacteria - metabolism; Actinobacteria - radiation effects; Bacterial Proteins - genetics; Bacterial Proteins - metabolism; Biosynthetic Pathways; Chromatography, High Pressure Liquid; Light; phylum Actinobacteria; Regular Paper; retinal biosynthesis; rhodopsin; Rhodopsin - biosynthesis; Rhodopsin - chemistry; Tandem Mass Spectrometry; rhodopsin; retinal biosynthesis; phylum Actinobacteria
• ISSN: 1342-6311; 1347-4405
• DOI: 10.1264/jsme2.ME20085

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.