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[Display omitted] • Cross-linking chemistry and mass spectrometry can be used for probing IDP/IDRs. • AlphaFold generated structures can be justified using experimental constraints. • The PB-loop is associated with ApcG, a novel linker protein in phycobilisome. • Potential roles of the PB-loop and ApcG in PBS biogensis. Intrinsically disordered proteins/regions (IDPRs) are a very large and functionally important class of proteins that participate in weak multivalent interactions in protein complexes. They are recalcitrant for interrogations using X-ray crystallography and cryo-EM. The IDPRs observed at the interface of the photosynthetic pigment protein complexes (PPCs) remain much less clear, e.g., the major cyanobacterial light-harvesting complex (PBS) contains an unstructured PB-loop insertion in the phycocyanobilin domain (PB domain) of ApcE (the largest polypeptide in PBS). Here, a joint platform is built to probe such structural domains. This platform is characterized by two-round progressive justifications of in silico models by using the structural mass spectrometry data. First, the AlphaFold-generated 3D structure of the PB domain (containing PB-loop) was justified in the context of PBS. Second, docking the AlphaFold-generated ApcG (a ligand) into the first-step justified structure (a receptor). The final ligand-receptor complex was then subjected to a second-round justification, again, by using unequivocal isotopically-encoded cross-links identified in LC-MS/MS. This work reveals a full-length PB-loop structure modelled in the PBS basal cylinder, free from any spatial conflicts against the other subunits in PBS. The structure of PB domain highlights the close associations of the intrinsically disordered PB-loop with its binding partners in PBS, including ApcG, another IDPR. The PB-loop region involved in the binding of photosystem II (PSII) is also discussed in the context of excitation energy transfer regulation. This work calls attention to the highly disordered, yet interrogatable interface between the light-harvesting antenna complexes and the reaction centers. [ABSTRACT FROM AUTHOR]

13 C metabolism analysis of a microbial community is often hindered by the time-consuming and complicated separation procedure for a single species. However, a “reporter protein,” produced uniquely by one cell type, retains 13 C fingerprint information in microbial consortia. This study describes the use of photosystem I (PSI), a multi-subunit protein complex universally found in oxygenic phototrophs, as a reliable reporter protein to probe microalgal metabolism (i.e., cyanobacterium Synechocystis sp. PCC 6803) in a mixed culture with heterotrophic bacteria (i.e., Escherichia coli ). We demonstrate that efficient purification of PSI and subsequent 13 C-based amino acid analyses may decipher photomixotrophic metabolism of Synechocystis 6803 in the coculture. This study also indicates that a supplement of NaHCO 3 at high concentration could significantly improve the robustness of cyanobacterial growth against bacterial contamination. [ABSTRACT FROM AUTHOR]

Phycobilisome (PBS) is a large pigment-protein complex in cyanobacteria and red algae responsible for capturing sunlight and transferring its energy to photosystems (PS). Spectroscopic and structural properties of various PBSs have been widely studied, however, the nature of so-called complex-complex interactions between PBS and PSs remains much less explored. In this work, we have investigated the function of a newly identified PBS linker protein, ApcG, some domain of which, together with a loop region (PB-loop in ApcE), is possibly located near the PBS-PS interface. Using Synechocystis sp. PCC 6803, we generated an ApcG deletion mutant and probed its deletion effect on the energetic coupling between PBS and photosystems. Steady-state and time-resolved spectroscopic characterization of the purified ΔApcG-PBS demonstrated that ApcG removal weakly affects the photophysical properties of PBS for which the spectroscopic properties of terminal energy emitters are comparable to those of PBS from wild-type strain. However, analysis of fluorescence decay imaging datasets reveals that ApcG deletion induces disruptions within the allophycocyanin (APC) core, resulting in the emergence (splitting) of two spectrally diverse subgroups with some short-lived APC. Profound spectroscopic changes of the whole ΔApcG mutant cell, however, emerge during state transition, a dynamic process of light scheme adaptation. The mutant cells in State I show a substantial increase in PBS-related fluorescence. On the other hand, global analysis of time-resolved fluorescence demonstrates that in general ApcG deletion does not alter or inhibit state transitions interpreted in terms of the changes of the PSII and PSI fluorescence emission intensity. The results revealed yet–to–be discovered mechanism of ApcG-docking induced excitation energy transfer regulation within PBS or to Photosystems. • Time-resolved fluorescence unveiled the photophysical characteristics of ΔApcG-PBS. • Mutation induces very minor disruptions within the allophycocyanin (APC) core. • Distinct spectral subgroups of short-lived APC identified in ΔApcG-PBS • Small linker protein causes altered energy transfer from PBS to RC. • Deletion of ApcG subunit does not impede state transitions in mutant cells. [ABSTRACT FROM AUTHOR]

Photosynthetic pigment-protein complexes (PPCs) accomplish light-energy capture and photochemistry in natural photosynthesis. In this review, we examine three pigment protein complexes in oxygenic photosynthesis: light-harvesting antenna complexes and two reaction centers: Photosystem II (PSII), and Photosystem I (PSI). Recent technological developments promise unprecedented insights into how these multi–component protein complexes are assembled into higher order structures and thereby execute their function. Furthermore, the interfacial domain between light-harvesting antenna complexes and PSII, especially the potential roles of the structural loops from CP29 and the PB–loop of ApcE in higher plant and cyanobacteria, respectively, are discussed. It is emphasized that the structural nuances are required for the structural dynamics and consequently for functional regulation in response to an ever–changing and challenging environment. • Structural similarities of higher plant LHC-PSII assembly and Cyanobacterial PBS-PSII assembly • The structural loop of higher plant CP29 and cyanobacterial PB-loop play similar functional roles mediating complex assembly • The PsbW-PsbI-CP43-D1-PsbO axis acts as a hub sensing proton status [ABSTRACT FROM AUTHOR]

Cyanobacteria inhabiting desert biological soil crusts face the harsh conditions of the desert. They evolved a suite of strategies toward desiccation-hydration cycles mixed with high light irradiations, etc. In this study we purified and characterized the structure and function of Photosystem I (PSI) from Leptolyngbya ohadii , a desiccation-tolerant desert cyanobacterium. We discovered that PSI forms tetrameric (PSI-Tet) aggregate. We investigated it by using sucrose density gradient centrifugation, clear native PAGE, high performance liquid chromatography, mass spectrometry (MS), time-resolved fluorescence (TRF) and time-resolved transient absorption (TA) spectroscopy. MS analysis identified the presence of two PsaB and two PsaL proteins in PSI-Tet and uniquely revealed that PsaLs are N-terminally acetylated in contrast to non-modified PsaL in the trimeric PSI from Synechocystis sp. PCC 6803. Chlorophyll (Chl) a fluorescence decay profiles of the PSI-Tet performed at 77 K revealed two emission bands at ∼690 nm and 725 nm with the former appearing only at early delay time. The main fluorescence emission peak, associated with emission from the low energy Chls a , decays within a few nanoseconds. TA studies demonstrated that the 725 nm emission band is associated with low energy Chls a with absorption band clearly resolved at ∼710 nm at 77 K. In summary, our work suggests that the heterogenous composition of PsaBs and PsaL in PSI-Tet is related with the adaptation mechanisms needed to cope with stressful conditions under which this bacterium naturally grows. • Leptolyngbya ohadii contains tetrameric Photosystem I (PSI). • Two types of PsaB and PsaL are identified in tetrameric PSI. • PsaL is N-terminally acetylated in this desiccation-tolerant cyanobacterium. • Spectral features of PSI are likely associated with the high degree of PSI aggregation. [ABSTRACT FROM AUTHOR]

Photosystem I (PSI) from oxygenicphotosynthetic organisms is anattractive sensitizer for nano-biohybrid solar cells as it has a combinedlight-harvesting and reaction center in one protein complex and operatesat a quantum yield close to one in biological systems. Using a linker-freedeposition technique enabled by an electrospray system, PSI was coupledto 1-D nanostructured titanium dioxide thin films to fabricate anelectrode for a photoelectrochemical cell. After deposition, the surfactantin the PSI aggregate was dissolved in the surfactant-free electrolyte,ensuring that partly hydrophobic PSI was not resuspended and stayedin contact with titanium dioxide. A maximum current density of 4.15mA cm–2was measured after 10 min of electrospraydeposition, and this is the highest current density reported so farfor PSI-based photoelectrochemical cells. The high current is attributedto 1D nanostructure of titanium dioxide and orientation of the PSIonto the surface, which allows easy transfer of electrons. [ABSTRACT FROM AUTHOR]