Deletion of sll1541 in Synechocystis sp. Strain PCC 6803 Allows Formation of a Far-Red-Shifted holo-Proteorhodopsin In Vivo

Two cyanobacterial species exhibit stress responses when grown together in visible light or far-red light

Although most cyanobacteria grow in visible light (VL; λ = 400-700 nm), some cyanobacteria can also use far-red light (FRL; λ = 700-800 nm) for oxygenic photosynthesis by performing far-red light photoacclimation. These two types of cyanobacteria can be found in the same environment. However, how they respond to each other remains unknown. Here, we reveal that coculture stresses FRL-using Chlorogloeopsis fritschii PCC 9212 and VL-using Synechocystis sp. PCC 6803. No significant growth difference was found in Synechocystis sp. PCC 6803 between the coculture and the monoculture. Conversely, the growth of Chlorogloeopsis fritschii PCC 9212 was suppressed in VL under coculture. According to transcriptomic analysis, Chlorogloeopsis fritschii PCC 9212 in coculture shows low transcript levels of metabolic activities and high transcript levels of ion transporters, with the differences being more noticeable in VL than in FRL. The transcript levels of stress responses in coculture were likewise higher than in monoculture in Synechocystis sp. PCC 6803 under FRL. The low transcript level of metabolic activities in coculture or the inhibition of cyanobacterial growth indicates a possible negative interaction between these two cyanobacterial strains.IMPORTANCEThe interaction between two cyanobacterial species is the primary focus of this study. One species harvests visible light, while the other can harvest far-red and visible light. Prior research on cyanobacteria interaction concentrated on its interactions with algal, coral, and fungal species. Interactions between cyanobacterial species were, nevertheless, rarely discussed. Thus, we characterized the interaction between two cyanobacterial species, one capable of photosynthesis using far-red light and the other not. Through experimental and bioinformatic approaches, we demonstrate that when one cyanobacterium thrives under optimal light conditions, it stresses the remaining cyanobacterial species. We contribute to an ecological understanding of these two kinds of cyanobacteria distribution patterns. Cyanobacteria that utilize far-red light probably disperse in environments with limited visible light to avoid competition with other cyanobacteria. From a biotechnological standpoint, this study suggests that the simultaneous cultivation of two cyanobacterial species in large-scale cultivation facilities may reduce the overall biomass yield.

Overexpressing Carotenoid Biosynthetic Genes in Synechocystis sp. PCC 6803 Improved Intracellular Pigments and Antioxidant Activity, Which Can Decrease the Viability and Proliferation of Lung Cancer Cells In Vitro

In the antioxidant system in cyanobacteria, non-enzymatic antioxidants, such as carotenoids, are considered good candidates for coping with oxidative stress, particularly light stress, and pharmaceutical therapeutic applications. A significant amount of carotenoid accumulation has been recently improved by genetic engineering. In this study, to achieve higher carotenoid production with higher antioxidant activity, we successfully constructed five Synechocystis sp. PCC 6803 strains overexpressing (OX) native genes related to the carotenoids biosynthetic pathway, including OX_CrtB, OX_CrtP, OX_CrtQ, OX_CrtO, and OX_CrtR. All of the engineered strains maintained a significant quantity of myxoxanthophyll, while increasing zeaxanthin and echinenone accumulation. In addition, higher components of zeaxanthin and echinenone were noted in all OX strains, ranging from 14 to 19% and from 17 to 22%, respectively. It is worth noting that the enhanced echinenone component responded to low light conditions, while the increased β-carotene component contributed to a high light stress response. According to the higher antioxidant activity of all OX strains, the carotenoid extracts presented lower IC₅₀ in lung cancer cell lines H460 and A549, with values less than 157 and 139 µg/mL, respectively, when compared with those of WTc, particularly OX_CrtR and OX_CrtQ. A higher proportion of zeaxanthin and β-carotene in OX_CrtR and OX_CrtQ, respectively, may considerably contribute to the ability to treat lung cancer cells with antiproliferative and cytotoxic effects.

Rhodopsins: An Excitingly Versatile Protein Species for Research, Development and Creative Engineering

The first member and eponym of the rhodopsin family was identified in the 1930s as the visual pigment of the rod photoreceptor cell in the animal retina. It was found to be a membrane protein, owing its photosensitivity to the presence of a covalently bound chromophoric group. This group, derived from vitamin A, was appropriately dubbed retinal. In the 1970s a microbial counterpart of this species was discovered in an archaeon, being a membrane protein also harbouring retinal as a chromophore, and named bacteriorhodopsin. Since their discovery a photogenic panorama unfolded, where up to date new members and subspecies with a variety of light-driven functionality have been added to this family. The animal branch, meanwhile categorized as type-2 rhodopsins, turned out to form a large subclass in the superfamily of G protein-coupled receptors and are essential to multiple elements of light-dependent animal sensory physiology. The microbial branch, the type-1 rhodopsins, largely function as light-driven ion pumps or channels, but also contain sensory-active and enzyme-sustaining subspecies. In this review we will follow the development of this exciting membrane protein panorama in a representative number of highlights and will present a prospect of their extraordinary future potential.

Genomic and transcriptomic evidence of light-sensing, porphyrin biosynthesis, Calvin-Benson-Bassham cycle, and urea production in Bathyarchaeota

BACKGROUND

Bathyarchaeota, a newly proposed archaeal phylum, is considered as an important driver of the global carbon cycle. However, due to the great diversity of them, there is limited genomic information that accurately encompasses the metabolic potential of the entire archaeal phylum.

RESULTS

In the current study, nine metagenome-assembled genomes of Bathyarchaeota from four subgroups were constructed from mangrove sediments, and metatranscriptomes were obtained for evaluating their in situ transcriptional activities. Comparative analyses with reference genomes and the transcripts of functional genes posit an expanded role for Bathyarchaeota in phototrophy, autotrophy, and nitrogen and sulfur cycles, respectively. Notably, the presence of genes for rhodopsins, cobalamin biosynthesis, and the oxygen-dependent metabolic pathways in some Bathyarchaeota subgroup 6 genomes suggest a light-sensing and microoxic lifestyle within this subgroup.

CONCLUSIONS

The results of this study expand our knowledge of metabolic abilities and diverse lifestyles of Bathyarchaeota, highlighting the crucial role of Bathyarchaeota in geochemical cycle. Video abstract.

Structural and mechanistic aspects of carotenoid cleavage dioxygenases (CCDs)

Carotenoid cleavage dioxygenases (CCDs) comprise a superfamily of mononuclear non-heme iron proteins that catalyze the oxygenolytic fission of alkene bonds in carotenoids to generate apocarotenoid products. Some of these enzymes exhibit additional activities such as carbon skeleton rearrangement and trans-cis isomerization. The group also includes a subfamily of enzymes that split the interphenyl alkene bond in molecules such as resveratrol and lignostilbene. CCDs are involved in numerous biological processes ranging from production of light-sensing chromophores to degradation of lignin derivatives in pulping waste sludge. These enzymes exhibit unique features that distinguish them from other families of non-heme iron enzymes. The distinctive properties and biological importance of CCDs have stimulated interest in their modes of catalysis. Recent structural, spectroscopic, and computational studies have helped clarify mechanistic aspects of CCD catalysis. Here, we review these findings emphasizing common and unique properties of CCDs that enable their variable substrate specificity and regioselectivity. This article is part of a Special Issue entitled Carotenoids recent advances in cell and molecular biology edited by Johannes von Lintig and Loredana Quadro.

Functional Expression of Gloeobacter Rhodopsin in PSI-Less Synechocystis sp. PCC6803

The approach of providing an oxygenic photosynthetic organism with a cyclic electron transfer system, i.e., a far-red light-driven proton pump, is widely proposed to maximize photosynthetic efficiency via expanding the absorption spectrum of photosynthetically active radiation. As a first step in this approach, Gloeobacter rhodopsin was expressed in a PSI-deletion strain of Synechocystis sp. PCC6803. Functional expression of Gloeobacter rhodopsin, in contrast to Proteorhodopsin, did not stimulate the rate of photoheterotrophic growth of this Synechocystis strain, analyzed with growth rate measurements and competition experiments. Nevertheless, analysis of oxygen uptake and—production rates of the Gloeobacter rhodopsin-expressing strains, relative to the ΔPSI control strain, confirm that the proton-pumping Gloeobacter rhodopsin provides the cells with additional capacity to generate proton motive force. Significantly, expression of the Gloeobacter rhodopsin did modulate levels of pigment formation in the transgenic strain.

Strong pH-Dependent Near-Infrared Fluorescence in a Microbial Rhodopsin Reconstituted with a Red-Shifting Retinal Analogue

Near-infrared (NIR)-driven rhodopsins are of great interest in optogenetics and other optobiotechnological developments such as artificial photosynthesis and deep-tissue voltage imaging. Here we report that the proton pump proteorhodopsin (PR) containing a NIR-active retinal analogue (PR:MMAR) exhibits intense NIR fluorescence at a quantum yield of 3.3%. This is 130 times higher than native PR ( Lenz , M. O. ; Biophys J. 2006 , 91 , 255 - 262 ) and 3-8 times higher than the QuasAr and PROPS voltage sensors ( Kralj , J. ; Science 2011 , 333 , 345 - 348 ; Hochbaum , D. R. ; Nat. Methods 2014 , 11 , 825 - 833 ). The NIR fluorescence strongly depends on the pH in the range of 6-8.5, suggesting potential application of MMAR-binding proteins as ultrasensitive NIR-driven pH and/or voltage sensors. Femtosecond transient absorption spectroscopy showed that upon near-IR excitation, PR:MMAR features an unusually long fluorescence lifetime of 310 ps and the absence of isomerized photoproducts, consistent with the high fluorescence quantum yield. Stimulated Raman analysis indicates that the NIR-absorbing species develops upon protonation of a conserved aspartate, which promotes charge delocalization and bond length leveling due to an additional methylamino group in MMAR, in essence providing a secondary protonated Schiff base. This results in much smaller bond length alteration along the conjugated backbone, thereby conferring significant single-bond character to the C13═C14 bond and structural deformation of the chromophore, which interferes with photoinduced isomerization and extends the lifetime for fluorescence. Hence, our studies allow for a molecular understanding of the relation between absorption/emission wavelength, isomerization, and fluorescence in PR:MMAR. As acidification enhances the resonance state, this explains the strong pH dependence of the NIR emission.