Biology subjects

Nawrocki, W.

Publications and source records attributed to Nawrocki, W..

3 recordsLinked to original sources

bioRxiv2025

A Unique LHCE Light-Harvesting protein Family is involved in Photosystem I and II Far-Red Absorption in Euglena gracilis

Photosynthetic organisms have evolved diverse strategies to adapt to fluctuating light conditions, balancing efficient light capture with photoprotection. In green algae and land plants, this involves specialized light-harvesting complexes (LHCs), non-photochemical quenching, and state transitions driven by dynamic remodeling of antenna proteins associated with Photosystems (PS) I and II. Euglena gracilis, a flagellate with a secondary green plastid, represents a distantly related lineage whose light-harvesting regulation remains poorly understood. Although spectral shifts under different light regimes have been observed, their molecular basis was unknown. Here, through integrated phylogenomic, proteomic, structural, and spectroscopic analyses, we identify a novel chlorophyll a far-red-absorbing antenna complex in E. gracilis, composed of a species-specific Lhce protein family. This antenna forms a pentameric complex under low light and transiently associates with PSII during far-red light exposure. It is structurally and functionally distinct from canonical LHCII{square} trimers and absent in Viridiplantae. Additionally, PSI in E. gracilis is surrounded by an expanded Lhce/LhcbM belt around a minimal core. These findings reveal a unique mechanism for regulating PS antenna size in E. gracilis, distinct from known models in plants and green algae, and highlight an alternative evolutionary strategy for light acclimation in organisms with secondary plastids. HighlightEuglena gracilis features a unique, lineage-specific LhcE antenna system that dynamically associates with PSII and expands PSI light harvesting, revealing an alternative strategy for light acclimation in secondary plastids.

plant biology↗

bioRxiv2017

Cyclic electron flow in Chlamydomonas reinhardtii

Cyclic electron flow (CEF), one of the major alternative electron transport pathways to the primary linear electron flow (LEF) in chloroplasts has been discovered in the middle of the last century. It is defined as a return of the reductants from the acceptor side of the Photosystem I (PSI) to the pool of its donors via the cytochrome b6f, and has proven essential for photosynthesis. However, despite many efforts aimed at its characterisation, the pathway and regulation of CEF remain equivocal, and its physiological significance remains to be properly defined. Here we use novel spectroscopic approaches to measure CEF in transitory conditions in the green alga Chlamydomonas reinhardtii. We show that CEF operates at the same maximal rate regardless of the oxygen concentration, and that the latter influences LEF, rather than CEF in vivo, which questions the recent hypotheses about the CEF supercomplex formation. We further reveal that the pathways proposed for CEF in the literature are inconsistent with the kinetic information provided by our measurements. We finally provide cues on the regulation of CEF by light.

plant biology↗

bioRxiv2017

Photosynthesis is heavily chlororespiration-sensitive under fluctuating light conditions

Photosynthesis needs to adjust to dynamically changing light intensities in order to maximize its efficiency, notably by the employment of alternative electron pathways. One of them is chlororespiration - initially described in Chlamydomonas reinhardtii. This electron transfer pathway, found in all photosynthetic lineages, consists of a reduction of plastoquinone (PQ) through an NAD(P)H:PQ oxidoreductase and quinol (PQH2) oxidation by Plastid Terminal Oxidase, PTOX. Hence, chlororespiration constitutes an electron pathway potentially antagonistic to the linear photosynthetic electron flow from H2O to CO2. However, the limited flow chlororespiratory enzymes can sustain suggests that their relative contribution, at least in the light and in steady-state conditions, is insubstantial. Here, we focused on the involvement of PTOX in Chlamydomonas reinhardtii during transitions from dark to light and vice versa. We show that the kinetics of redox relaxation of the chloroplast in the dark was greatly affected when PTOX2, the major plastoquinol oxidase in Chlamydomonas, is lacking. Importantly, we show that this has a direct physiological relevance, as the growth of a PTOX2-lacking mutant is markedly slower in intermittent light. The latter can be rationalized in terms of a decreased flux sustained by photosystem II due to a redox limitation at the acceptor side of the PSI during the illumination periods. We finally show that the long-term regulation of cyclic electron flow around PSI is strongly affected in the PTOX2 mutant, substantiating an important role of chlororespiration in the maintenance of chloroplast redox balance.

plant biology↗