Abstract
Microbial rhodopsin is a simple solar energy-capturing molecule compared to the complex photosynthesis apparatus. Light-driven proton pumping across the cell membrane is a crucial mechanism underlying microbial energy production. Actinobacteria is one of the highly abundant bacterial phyla in freshwater habitats, and members of this lineage are considered to boost heterotrophic growth via phototrophy, as indicated by the presence of actino-opsin (ActR) genes in their genome. However, it is difficult to validate their function under laboratory settings because Actinobacteria are not consistently cultivable. Based on the published genome sequence of Candidatus aquiluna sp. strain IMCC13023, actinorhodopsin from the strain (ActR-13023) was isolated and characterized in this study. Notably, ActR-13023 assembled with natively synthesized carotenoid/retinal (used as a dual chromophore) and functioned as a light-driven outward proton pump. The ActR-13023 gene and putative genes involved in the chromophore (retinal/carotenoid) biosynthetic pathway were detected in the genome, indicating the functional expression ActR-13023 under natural conditions for the utilization of solar energy for proton translocation. Heterologous expressed ActR-13023 exhibited maximum absorption at 565 nm with practical proton pumping ability. Purified ActR-13023 could be reconstituted with actinobacterial carotenoids for additional light-harvesting. The existence of actinorhodopsin and its chromophore synthesis machinery in Actinobacteria indicates the inherent photo-energy conversion function of this microorganism. The assembly of ActR-13023 to its synthesized chromophores validated the microbial community’s importance in the energy cycle.
Highlights
Actinobacteria are found in various terrestrial and aquatic environments
The protein sequence alignment of ActR-13023 to BR, PR, and XR from S. ruber (PDB: 3ddl) that was reported for carotenoid interaction (Luecke et al, 2008), and other actinorhodopsins (Dwulit-Smith et al, 2018) showed that ActR-13023 shared 11 out of 18 residues that were reported for carotenoid interaction in XR
The high abundance of actinobacteria in aquatic habitats, seawater, sea ice, and freshwater has led to establishing a cellular adaptation model for energy scavenging fitness
Summary
Actinobacteria are found in various terrestrial and aquatic environments. They are the most morphologically diverse prokaryotes (Servin et al, 2008). The Actinobacteria clade (OM1 clade; Rappé et al, 1998), constituted by members of the class Actinobacteria, had been recognized as a contributor to marine microbial communities in culture-independent studies, which involved 16S rRNA gene pyrosequencing, metagenome sequencing, and metaproteomic experiments (Jensen and Lauro, 2008). The Actinobacteria strain IMCC13023 was isolated from a surface seawater sample collected from Kongsfjorden (Svalvard, Norway) during the glacier-melting season using highthroughput extinction-to-dilution culturing. A protein-centric comparative metaproteomic approach adopted on an oceanic scale, targeting membrane proteins, suggested the involvement of the bacteria in nutrient transport and energy transduction, which altered our understanding of the bacterial community structure, nutrient utilization, and energy transduction, as well as viral and archaeal activities (Morris et al, 2010)
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