Biology subjects

Ostermeier, M.

Publications and source records attributed to Ostermeier, M..

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↗

bioRxiv2026

CurT dosage quantitatively affects thylakoid structure and PSII performance in Synechocystis sp. PCC 6803

The photosynthetic thylakoid membrane systems of many oxygenic photosynthesizers form elaborate three-dimensional structures. Curvature Thylakoid (CurT/CURT1) proteins are central regulators of thylakoid architecture in cyanobacteria and plants, driving both grana stacking and the formation of thylakoid convergence zones (TCZs) by promoting membrane curvature while also contributing to cyanobacterial cell and green-algal chloroplast division. While the functional role of grana stacks in terrestrial photosynthesis is well established, the physiological significance of cyanobacterial TCZs remains unclear. Here, we investigated the role of TCZs in Synechocystis sp. PCC 6803 by quantitatively assessing how CurT protein abundance shapes thylakoid membrane organization, TCZ frequency, and thylakoid layering, and how these architectural features relate to photosynthetic efficiency in a set of curT expression mutants. By correlating defined structural parameters of the thylakoid system with cellular CurT levels, we defined for the first time the quantitative relationship between CurT abundance, thylakoid architecture, and photosystem II (PSII) performance. Our results reveal a non-linear, logarithmic relationship: minimal CurT suffices to restore TCZ formation and sustain WT-like growth, while overexpression further enhances PSII activity and culture yield. Together, these findings identify CurT as a dose-dependent key determinant of thylakoid structure and underscore the functional importance of thylakoid membrane architecture for efficient photosynthesis. Significance StatementThis study assesses how the membrane-shaping protein CurT quantitatively regulates thylakoid architecture and photosynthetic performance in the model cyanobacterium Synechocystis sp. PCC 6803. By relating CurT abundance to the formation of thylakoid convergence zones, photosystem II efficiency, and growth rate, we provide evidence of CurT acting as a key regulator of cyanobacterial membrane organization in a dosage-dependent manner. Our findings provide insight into the structural basis and physiological relevance of thylakoid convergence zones and suggest CurT as a potential target for improving cyanobacterial light-energy conversion.

microbiology↗

bioRxiv2026

Environmental and mutational modulation of collateral fitness effects informs their mechanisms

Fitness effects of mutations that do not arise from changes in a proteins ability to perform its physiological functions (called collateral fitness effects or CFEs) are an understudied aspect of fitness landscapes. We have previously systematically measured the CFEs of all possible single amino acid substitutions in four proteins and found the frequency of deleterious mutations to vary by two orders of magnitude. Of these proteins, TEM-1 {beta}-lactamase had the highest frequency, and deleterious mutations caused TEM-1 aggregation. Here, we systematically measured TEM-1 collateral fitness landscapes in environments and situations expected to alter protein aggregation or protein stability. We found a moderate correlation between deleterious CFEs and predicted thermodynamic stability effects in TEM-1s -domain. Empirically, we found that the frequency and magnitude of deleterious CFEs can be reduced by altering the growth environment to disfavor aggregation (i.e. reducing the growth temperature or shifting to minimal media) or by stabilizing TEM-1 (via the M182T mutation or the addition of the {beta}-lactamase inhibitor avibactam to the growth medium). However, although raising the growth temperature to favor aggregation exacerbated deleterious CFEs of many mutations, many mutations effects were reduced. Furthermore, although reductions in CFEs occurred with reductions in TEM-1 aggregation for some mutants, for many mutants they did not. We propose that mutational destabilization exposes protein motifs that can cause deleterious CFEs, but that these motifs and those that cause aggregation are not necessarily the same motifs.

evolutionary biology↗

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

Fitness and functional landscapes of the E. coli RNase III gene rnc

How protein properties such as protein activity and protein essentiality affect the distribution of fitness effects (DFE) of mutations are important questions in protein evolution. Deep mutational scanning studies typically measure the effects of a comprehensive set of mutations on either protein activity or fitness. Our understanding of the underpinnings of the DFE would be enhanced by a comprehensive study of both for the same gene. Here, we compared the fitness effects and in vivo protein activity effects of [~]4,500 missense mutations in the E. coli rnc gene. This gene encodes RNase III, a global regulator enzyme that cleaves diverse RNA substrates including precursor ribosomal RNA and various mRNAs including its own 5 untranslated region (5UTR). We find that RNase IIIs ability to cleave dsRNA is the most important determinant of the fitness effects of rnc mutations. The DFE of RNase III was bimodal, with mutations centered around neutral and deleterious effects, consistent with previously reported DFEs of enzymes with a singular physiological role. Fitness was buffered to small effects on RNase III activity. The enzymes RNase III domain (RIIID), which contains the RNase III signature motif and all active site residues, was more sensitive to mutation than its dsRNA binding domain (dsRBD), which is responsible for recognition and binding to dsRNA. Differential effects on fitness and functional scores for mutations at highly conserved residues G97, G99, and F188 suggest that these positions may be important for RNase III cleavage specificity.

evolutionary biology↗

bioRxiv2022

Genes vary greatly in their propensity for collateral fitness effects of mutations

Mutations can have deleterious fitness effects when they decrease protein specific activity or decrease active protein abundance. Mutations will also be deleterious when they cause misfolding or misinteractions that are toxic to the cell (i.e., independent of whether the mutations affect specific activity and abundance). The extent to which protein evolution is shaped by these and other collateral fitness effects is unclear in part because little is known of their frequency and magnitude. Using deep mutational scanning (DMS), we previously found at least 42% of missense mutations in the TEM-1 {beta}-lactamase antibiotic resistance gene cause deleterious collateral fitness effects. Here, we used DMS to comprehensively determine the collateral fitness effects of missense mutations in three genes encoding the antibiotic resistance proteins New Delhi metallo-{beta}-lactamase (NDM-1), chloramphenicol acetyltransferase I (CAT-I), and 2"-aminoglycoside nucleotidyltransferase (AadB). AadB (20%), CAT-I (0.9%), and NDM-1 (0.2%) were less susceptible to deleterious collateral fitness effects than TEM-1 (42%) indicating that genes have different propensities for these effects. As was observed with TEM-1, all the studied deleterious aadB mutants increased aggregation. However, aggregation did not correlate with collateral fitness effects for many of the deleterious mutants of CAT-I and NDM-1. Select deleterious mutants caused unexpected phenotypes to emerge. The introduction of internal start codons in CAT-1 caused loss of the episome and a mutation in aadB made its cognate antibiotic essential for growth. Our study illustrates how the complexity of the cell provides a rich environment for collateral fitness effects and new phenotypes to emerge.

evolutionary biology↗

bioRxiv2019

Collateral fitness effects of mutations

The distribution of fitness effects (DFE) of mutation plays a central role in constraining protein evolution. The underlying mechanisms by which mutations lead to fitness effects are typically attributed to changes in protein specific activity or abundance. Here, we reveal the importance of a mutations collateral fitness effects, which we define as effects that do not derive from changes in the proteins ability to perform its physiological function. We comprehensively measured the collateral fitness effects of missense mutations in the E. coli TEM-1 {beta}-lactamase antibiotic resistance gene using growth competition experiments in the absence of antibiotic. At least 42% of missense mutations in TEM-1 were deleterious, indicating that for some proteins, collateral fitness effects occur as frequently as effects on protein activity and abundance. Deleterious mutations caused improper post-translational processing, incorrect disulfide-bond formation, protein aggregation, changes in gene expression, and pleiotropic effects on cell phenotype. Deleterious collateral fitness effects occurred more frequently in TEM-1 than deleterious effects on antibiotic resistance in environments with low concentrations of the antibiotic. The surprising prevalence of deleterious collateral fitness effects suggests they may play a role in constraining protein evolution, particularly for highly-expressed proteins, for proteins under intermittent selection for their physiological function, and for proteins whose contribution to fitness is buffered against mutations with deleterious effects on protein activity and protein abundance. Significance StatementMutations provide the source of genetic variability upon which evolution acts. Deleterious protein mutations are commonly thought of in terms of how they compromise the proteins ability to perform its physiological function. However, mutations might also be deleterious if they cause negative effects on one of the countless other cellular processes. The frequency and magnitude of such collateral fitness effects is unknown. Our systematic study of mutations in a bacterial protein finds widespread collateral fitness effects that were associated with protein aggregation, improper protein processing, incomplete protein transport across membranes, incorrect disulfide-bond formation, induction of stress-response pathways, and unexpected changes in cell properties. Our results suggest that deleterious collateral fitness effects may be an important constraint on protein evolution.

evolutionary biology↗

bioRxiv2019

Protein-surfactant-polysaccharide nanoparticles increase the catalytic activity of an engineered β-lactamase maltose-activated switch enzyme

We present polysaccharide-based nanoparticles able to associate and increase the catalytic activity of the maltose-binding MBP317-347 switch enzyme. Fluorescence quenching and molecular docking studies along with the partial resistance to increasing pH and ionic strength indicate that the increase in enzymatic activity is due to a specific interaction between the maltose binding pocket on MBP317-347 and alginate exposed on the surface of the nanoparticles. Finally, we show that the hybrid self co-assembled particles increase the half-life of MBP317-347 over six-fold at 37{degrees}C, thus reflecting their potential use as a macromolecular drug delivery system.

biochemistry↗