Biology subjects

Nickelsen, J.

Publications and source records attributed to Nickelsen, J..

8 recordsLinked to original sources

bioRxiv2026

Tripartite synergy - Metabolic crosstalk between two bacterial mutualists and a marine microalga promotes algal fitness

Marine microalgae form major parts of phytoplankton and are highly relevant for global CO2 fixation. Although microalgae have lived together with bacteria in the oceans for billions of years, these ecosystem-relevant interactions remain largely uncharacterized. Here, we have studied biotic interactions between two marine bacteria and a marine microalga. We show that an N2-fixing Vibrio provides ammonium for Chlamydomonas sp. and a Marinobacterium. Both microorganisms cannot survive in an ammonium-free environment. In exchange, the microalga promotes the growth of both bacteria, via secretion of heat-resistant metabolites in case of Marinobacterium. Reciprocally, the Marinobacterium releases heat-resistant metabolites that stimulate algal growth and increase its photosynthetic pigments, Photosystem II quantum yield, and starch accumulation. Electron microscopy reveals a strengthened starch sheath around the algal pyrenoid and indicates a modified periplasmic space for metabolic exchange. Our data highlight a tight synergy of a marine microbial trio promoting each others growth and algal fitness.

microbiology↗

bioRxiv2025

Conserved and Lineage-Specific Roles of KEA-Mediated Ion Homeostasis in Chlamydomonas

In Arabidopsis thaliana, seamless plastid gene expression and development depend on finely balanced ion homeostasis across the inner envelope (IE) membrane, maintained by the K/H antiporters AtKEA1/2. To assess whether these functions are retained across mono- and polyplastidic representatives of the green lineage, we studied CrKEA1, the sole IE KEA homolog in the unicellular alga Chlamydomonas reinhardtii. Using CRISPR/Cas9, we generated a Cr-kea1 knockout mutant that exhibits impaired photoautotrophic growth, chloroplast deformation, and photoinhibition. Transcriptomics revealed strong induction of ribosome biogenesis genes and reduced abundance of transcripts associated with cell and plastid division. Further RNA analyses confirmed defects in stromal rRNA maturation of Cr-kea1, paralleling observations from Arabidopsis kea1kea2 mutants. Expression of CrKEA1 in Arabidopsis rescued growth and rRNA maturation in At-kea1kea2, demonstrating functional continuity after the ancient divergence between the two lineages. Cross-species transcriptomic comparisons further revealed that IE KEA loss elicits both shared and species-specific transcriptional responses: PhANG repression was conserved between algae and plants, whereas activation of the chloroplast unfolded protein response (cpUPR) and reduced expression of genes tied to cell-cycle and plastid fission occurred only in Chlamydomonas. Single-cell time-lapse imaging confirmed that Cr-kea1 exhibits an increased frequency of aberrant cytokinesis, unequal division, and division failure. Our findings demonstrate that while IE KEA transporters fulfill conserved roles in maintaining the conditions for plastid gene expression, their integration into broader cellular networks has diverged between unicellular chlorophytes and embryophytes (land plants). This underscores a lineage-dependent tuning of plastid-nucleus communication shaped by organismal complexity and plastid number. One-sentence summaryDisruption of KEA-mediated chloroplast ion homeostasis in Chlamydomonas reinhardtii reveals conserved and lineage-specific control of plastid rRNA processing and cell cycle progression.

plant biology↗

bioRxiv2022

OHP2 is not required for psbA translation in Chlamydomonas

In land plants and cyanobacteria, co-translational association of chlorophyll (Chl) to the nascent D1 polypeptide, a reaction center protein of photosystem II (PSII), requires a Chl binding complex consisting of a short-chain dehydrogenase (HCF244/Ycf39) and One-Helix Proteins of the LHC superfamily (OHP1 and OHP2 in chloroplasts). Here, we show that an ohp2 mutant of the green alga Chlamydomonas reinhardtii fails to accumulate core PSII subunits, in particular D1. Extragenic suppressors arise at high frequency, suggesting the existence of another route for Chl association to PSII. The ohp2 mutant can be complemented by the Arabidopsis ortholog. In contrast to land plants, where psbA translation is prevented in the absence of OHP2, ribosome profiling experiments show that the Chlamydomonas mutant translates the psbA transcript over its full length. Pulse labelling suggests that D1 is degraded during or immediately after translation. The translation of other PSII subunits is affected by assembly-controlled translational regulation (the CES process). Proteomics show that HCF244, a translation factor which associates with and is stabilized by OHP2 in land plants, still partly accumulates in the Chlamydomonas ohp2 mutant, explaining the persistence of psbA translation. Several Chl biosynthesis enzymes overaccumulate in the mutant membranes. Partial inactivation of the D1-degrading FtsH protease restores a low level of PSII activity in an ohp2 background, but not photoautotrophy. Taken together, our data suggest that OHP2 is not required for psbAD1 translation in Chlamydomonas, but necessary for its stabilization.

plant biology↗

bioRxiv2020

Structural basis for VIPP1 oligomerization and maintenance of thylakoid membrane integrity

Vesicle-inducing protein in plastids (VIPP1) is essential for the biogenesis and maintenance of thylakoid membranes, which transform light into life. However, it is unknown how VIPP1 performs its vital membrane-shaping function. Here, we use cryo-electron microscopy to determine structures of cyanobacterial VIPP1 rings, revealing how VIPP1 monomers flex and interweave to form basket-like assemblies of different symmetries. Three VIPP1 monomers together coordinate a non-canonical nucleotide binding pocket that is required for VIPP1 oligomerization. Inside the rings lumen, amphipathic helices from each monomer align to form large hydrophobic columns, enabling VIPP1 to bind and curve membranes. In vivo point mutations in these hydrophobic surfaces cause extreme thylakoid swelling under high light, indicating an essential role of VIPP1 lipid binding in resisting stress-induced damage. Our study provides a structural basis for understanding how the oligomerization of VIPP1 drives the biogenesis of thylakoid membranes and protects these life-giving membranes from environmental stress.

plant biology↗

bioRxiv2020

The chloroplast ribonucleoprotein CP33B quantitatively binds the psbA mRNA

Chloroplast RNAs are stabilized and processed by a multitude of nuclear-encoded RNA binding proteins, often in response to external stimuli like light and temperature. A particularly interesting RNA based regulation occurs with the psbA mRNA, which shows light-dependent translation. Recently, the chloroplast ribonucleoprotein CP33B was identified as a ligand of the psbA mRNA. We here characterized the interaction of CP33B with chloroplast RNAs in greater detail using a combination of RIP-chip, quantitative dot-blot, and RNA-Bind-n-Seq experiments. We demonstrate that CP33B prefers psbA over all other chloroplast RNAs and associates with vast majority of the psbA transcript pool. The RNA sequence target motif determined in vitro does not fully explain CP33Bs preference for psbA, suggesting that there are other determinants of specificity in vivo.

molecular biology↗

bioRxiv2019

The proteomic inventory reveals the chloroplast ribosome as nexus within a diverse protein network

Chloroplast gene expression is tightly regulated and majorly controlled on the level of protein synthesis. Fine-tuning of translation is vital for plant development, acclimation to environmental challenges and for the assembly of major protein complexes such as the photosynthesis machinery. However, many regulatory mediators and the interaction network of chloroplast ribosomes are not known to date. We report here on a deep proteomic analysis of the plastidic ribosome interaction network in Chlamydomonas reinhardtii cells. Affinity-purification of ribosomes was achieved via endogenous affinity tagging of the chloroplast-encoded protein Rpl5, yielding a specific enrichment of >650 chloroplast-localized proteins. The ribosome interaction network was validated for several proteins and provides a new source of mainly conserved factors directly linking translation with central processes such as protein folding, photosystem biogenesis, redox control, RNA maturation, energy and metabolite homeostasis. Our approach provided the first evidence for the existence of a plastidic co-translational acting N-acetyltransferase (cpNAT1). Expression of tagged cpNAT1 confirmed its ribosome-association, and we demonstrated the ability of cpNAT1 to acetylate substrate proteins at their N-terminus. Our dataset establishes that the chloroplast protein synthesis machinery acts as nexus in a highly choreographed, spatially interconnected protein network and underscores its wide-ranging regulatory potential during gene expression. ONE-SENTENCE SUMMARYAffinity purification of Chlamydomonas reinhardtii chloroplast ribosomes and subsequent proteomic analysis revealed a broad spectrum of interactors ranging from global translation control to specific pathways.

biochemistry↗

bioRxiv2019

RNA recognition by the E2 subunit of the chloroplast pyruvate dehydrogenase from Chlamydomonas

The dihydrolipoamide acetyltransferase subunit (DLA2) of the chloroplast puruvate dehydrogenase complex (cpPDC) from the green alga Chlamydomonas reinhardtii has previously been shown to possess a moonlighting activity in chloroplast gene expression. Exclusively under mixotrophic growth conditions, DLA2 forms part of an RNP particle with the psbA mRNA that encodes the D1 protein of the photosystem II reaction center. Here, we report on the further characterization of DLA2s RNA-binding activity. Size-exclusion chromatography and Western analyses revealed that DLA2 is the only cpPDC subunit that shuttles between the metabolic cpPDC and the RNP complex. Microscale thermophoresis-based determination of RNA-binding affinities demonstrated that two DLA2 regions are crucial for RNA recognition, the peripheral E3-binding domain (E3BD) and the C-terminus of the catalytic domain. Specificity for the psbA RNA probe is conferred by the E3BD in vitro, as verified by competitive binding assays in the presence of an excess of the E3 (DLD2) of cpPDC. The data support a model in which an environmental trigger induces release of DLA2 from the cpPDC and its subsequent association with the psbA mRNA.

plant biology↗

bioRxiv2019

Towards a biotechnological platform for the production of human growth factors in the green alga Chlamydomonas reinhardtii

The recent use of photosynthetic organisms such as Chlamydomonas reinhardtii in biomedical applications has demonstrated their potential for the treatment of acute and chronic tissue hypoxia. Moreover, transgenic microalgae have been suggested as an alternative in situ drug delivery system. In this study, we set out to identify the best available combination of strains and expression vectors to establish a robust platform for the expression of human pro-angiogenic growth factors, i.e. hVEGF-165, hPDGF-B, and hSDF-1, in biomedical settings. As a case study, combinations of two expression vectors (pOpt and pBC1) and two C. reinhardtii strains (UVM4 and UVM11) were compared with respect to hVEGF-165 transgene expression by determination of steady-state levels of transgenic transcripts as well as immunological detection of recombinant proteins produced and secreted by the generated strains. The results revealed the combination of the UVM11-strain with the pBC1-vector to be the most efficient one for high level hVEGF-165 production. To assess the robustness of this finding, the selected combination was used to create hPDGF-B and hSDF-1 transgenic strains for optimized recombinant protein expression. Furthermore, biological activity and functionality of algal-produced recombinant pro-angiogenic growth factors were assessed by receptor phosphorylation and in-vitro angiogenesis assays. The results obtained revealed a potentiating effect in the combinatorial application of transgenic strains expressing either of the three growth factors on endothelial cell tube formation ability, and thus support the idea of using transgenic algae expressing pro-angiogenic growth factors in wound healing approaches.

plant biology↗