Biology subjects

Santos-Merino, M.

Publications and source records attributed to Santos-Merino, M..

7 recordsLinked to original sources

bioRxiv2026

A novel class of conserved sucrose-phosphate phosphatases highlights the diversity of cyanobacterial sucrose metabolism

Sucrose metabolism is an important feature of the physiology of the green lineage of photosynthetic organisms and has therefore been the subject of considerable research on plants, algae, and cyanobacteria. Canonical sucrose biosynthesis pathways often involve the condensation of NDP-glucose and fructose-6-phosphate through the action of sucrose-phosphate synthase, then the dephosphorylation of sucrose 6-phosphate into sucrose via sucrose-phosphate phosphatase (SPP). However, many cyanobacterial genomes encode multiple homologs of SPP proteins (SPP-like), including variants that appear to lack key residues reported to be important for sucrose 6-phosphate binding. Herein, we examine these SPP-like proteins, focusing on the biochemical and physiological characterization of the SPP-like protein encoded by the cyanobacterial model, Synechococcus elongatus PCC 7942. Bioinformatic analysis suggests that the SPP-like family of proteins is highly conserved across cyanobacterial species and forms distinct phylogenetic clades that are more widely distributed than the canonical SPP proteins themselves. Biochemical analysis of the purified S. elongatus PCC 7942 SPP-like protein reveals that it not only retains the capacity to dephosphorylate sucrose 6-phosphate, but it also may have physiologically relevant phosphatase activity on 3-phosphoglycerate (3-PGA). We provide evidence that the SPP-like family of proteins represents a well-conserved group of phosphatases across cyanobacteria and suggest some enzymes in this family may have evolved a broader substrate specificity relative to the well-characterized SPP family. Our results have broader implications for cyanobacterial regulation of sucrose biosynthesis and other key steps of central carbon metabolism.

biochemistry↗

bioRxiv2025

HIGH CO2 LEVELS OVERCOME THE NIGHTTIME LETHALITY OF CIRCADIAN MUTANTS IN CYANOBACTERIA

Cyanobacteria have adapted to daily fluctuations in light intensity through a circadian clock that aligns their physiology and metabolism to the external daytime. In the model cyanobacterium Synechococcus elongatus PCC 7942, the rhythm generated by a molecular pacemaker is converted into a global transcriptional oscillation by the central regulator RpaA. Mutants in RpaA and other components have been instrumental in elucidating the clocks molecular architecture and physiological role. Extensive evidence suggests that one of the central functions of circadian regulation is to prevent the accumulation of reactive oxygen species (ROS) during nighttime. However, circadian mutants are unable to grow under natural day/night cycles and require constant illumination, hindering our ability to study the clock function during the night, a phase where circadian regulation is critical for redox homeostasis. Here, we show that the darkness lethality phenotype of circadian mutants can be overcome by high CO2 levels. When grown under a 3% CO2 atmosphere, RpaA-null mutants exhibited growth rates similar to the wt. An analysis of the ROS levels under different CO2 and light intensity conditions revealed carbon scarcity to be the most significant contributor to redox stress. Nighttime ROS accumulation can be modulated by CO2 abundance, an observation that will allow the characterization of heretofore lethal mutants in circadian regulation. The dispensability of the circadian clock in a high CO2 environment suggests that the clock may have evolved as an adaptation to the decrease in atmospheric CO2 levels that occurred after the Great Oxygenation Events, in the Paleoproterozoic era. IMPORTANCECyanobacterial circadian clocks are considered essential for survival under natural day/night conditions, and many circadian mutants are lethal under diel cycles. Results presented here challenge this assumption, showing that these mutants grow normally when CO2 is abundant. This suggests that the major adaptive role of the clock is to manage the redox stress caused by carbon limitation. By separating the effects of carbon availability from circadian control, we identify a key environmental factor that shaped the evolution of biological timekeeping. Using high CO2 to bypass clock dependency will allow the study of circadian function in otherwise lethal mutants. These findings reframe the clock as a specific adaptation to atmospheric carbon, stressing its key role in the regulation of the redox balance.

microbiology↗

bioRxiv2025

Autoinduction expression systems by AHL-mediated quorum-sensing circuits in Synechococcus elongatus PCC 7942

Despite significant potential for cyanobacteria as sustainable bioproduction chases, there are limited examples of scaled cyanobacterial bioproduction. In part, this is because most cyanobacterial species are poorly adapted to bioreactor cultivation conditions and lack features that facilitate biomass growth and harvesting at scale. We explored quorum sensing (QS) pathways derived from heterotrophic microbes as a method for autoinduction of gene expression circuits coordinated to population density in cyanobacteria. Here, we integrated genetic modules designed to produce and detect the diffusible QS signal, acyl-homoserine lactones (AHLs), in the cyanobacterial model, Synechococcus elongatus PCC 7942 (S. elongatus). We demonstrate that S. elongatus heterologously produces sufficient AHL signals to activate gene expression in a dose-dependent and population density-responsive manner. A hybrid combination of AHL synthesis enzyme from Vibrio fischeri (Lux system) with the transcription factor receiver from Pseudomonas aeruginosa (Las system) provides an ideal activation ratio and mitigates toxicity observed with some AHL systems. As a proof of concept, we coupled the QS pathway to the expression of a cell division inhibitory gene, cdv3, facilitating late-phase cell elongation, cell sedimentation, and improved biomass recovery. Our findings provide a foundation for the development of auto-induction systems leverageable to improve cyanobacterial biotechnology applications.

microbiology↗

bioRxiv2025

Plastoquinone redox status influences carboxysome integrity via a RpaA- and ROS-dependent regulatory network

Carboxysomes are bacterial microcompartments that encapsulate Rubisco and are a core component of the cyanobacterial carbon concentration mechanism (CCM). While carboxysome number, size and spatial organization are observed to vary in different environmental conditions (CO2, light, temperature, light quality), molecular mechanisms underlying this potentially adaptive process remain elusive. Herein, we observed that mutants of the circadian rhythm/metabolism factor, Regulator of Phycobilisome Associated A (RpaA), exhibit a striking breakdown of carboxysomes under certain environmental conditions. We find that growth conditions leading to overreduction of the plastoquinone (PQ) pool (mixotrophic growth, high irradiance, or chemical inhibition of electron transfer from PQ to the cytochrome b6f complex) are accompanied by elevated generation of reactive oxygen species (ROS), and correlate with carboxysome breakdown. Carboxysome breakdown is reversed by environmental conditions or chemical inhibitors that prevent PQ overreduction and accompanying ROS generation. Taken together, our data supports a novel link between the redox status of the PQ pool and carboxysome status and/or integrity. Our results have implications for fundamental understanding of cyanobacterial energy balancing pathways and may indicate new research directions for understanding how the carboxysome is remodeled in response to changing environments.

synthetic biology↗

bioRxiv2024

Long-term evolution reveals the role of the circadian cycle in the environmental adaptation of cyanobacteria

Circadian clocks synchronize internal cellular states with diurnal rhythms. Widespread in bacteria and eukaryotes, they regulate a variety of physiological processes, from hormone secretion in animals to carbon fixation in photosynthetic organisms. The adaptive role of circadian clocks is assumed to stem from their ability to anticipate environmental change, yet their impact on ecological adaptation remains unclear. Here, we use experimental evolution to study the interplay between fitness and circadian regulation in the model cyanobacterium Synechococcus elongatus PCC 7942. After 1,200 generations under continuous, high-intensity illumination, we obtained a strain that grew six times faster than its ancestral counterpart. Genome sequencing revealed three mutations fixed in the population, two of which replicated the fast-growing phenotype in the wild-type. A deletion in SasA, a key circadian regulator, was essential for fast growth. Transcriptomic and metabolomic analyses revealed that this mutation perturbed the rhythmicity of the cycle, while simultaneously locking the cell in a transcriptomic response to high intensity illumination. A comparison with another fast- growing isolate, UTEX 2973, showed convergent transcriptomic states despite different driving mutations. Our results indicate that the clock acts not only as a timekeeping device, but also as an adaptive mechanism to optimize growth across diverse ecological conditions.

microbiology↗

bioRxiv2022

Predicting partner fitness based on spatial structuring in a light-driven microbial community

Microbial communities have vital roles in systems essential to human health and agriculture, such as gut and soil microbiomes, and there is growing interest in engineering designer consortia for applications in biotechnology (e.g., personalized probiotics, bioproduction of high-value products, biosensing). The capacity to monitor and model metabolite exchange in dynamic microbial consortia can provide foundational information important to understand the community level behaviors that emerge, a requirement for building novel consortia. Where experimental approaches for monitoring metabolic exchange are technologically challenging, computational tools can enable greater access to the fate of both chemicals and microbes within a consortium. In this study, we developed an in-silico model of a synthetic microbial consortia of sucrose-secreting Synechococcus elongatus PCC 7942 and Escherichia coli W. Our model was built on the NUFEB framework for Individual-based Modeling (IbM) and optimized for biological accuracy using experimental data. We showed that the relative level of sucrose secretion regulates not only the steady-state support for heterotrophic biomass, but also the temporal dynamics of consortia growth. In order to determine the importance of spatial organization within the consortium, we fit a regression model to spatial data and used it to accurately predict colony fitness. We found that some of the critical parameters for fitness prediction were inter-colony distance, initial biomass, induction level, and distance from the center of the simulation volume. We anticipate that the synergy between experimental and computational approaches will improve our ability to design consortia with novel function. Author SummaryMicrobial communities, play important, yet poorly understood roles in health and agriculture. As we develop a better understanding of how these communities interact together, as well as with their host organisms, there is growing interest in engineering communities with specific functions, such as for treating disease, personalized probiotics, or aiding plants with nutrient uptake. To better understand how these microbes interact with each other, we want to monitor the exchange of metabolites and the locations of the microbes, tasks which at present are technically challenging, if not impossible. Where experimental approaches for monitoring metabolites are limited, computational tools can enable greater access to the fate of both chemicals and microbes within a community. In this study, we developed a computerized model of a synthetic microbial community of two bacteria, one which performs photosynthesis and supplies sugar and another which consumes the sugar for growth. We showed that the relative level of sugar secretion regulates not only the steady-state support for the consumer partners growth, but also how the community changes with time. To determine the importance of spatial organization within the community, we fit a model and used it to predict colony growth. We anticipate that the synergy between experimental and computational approaches will improve our ability to design microbial communities with new functions.

bioengineering↗

bioRxiv2021

Multi-layer Regulation of Rubisco in Response to Altered Carbon Status in Synechococcus elongatus PCC 7942

Photosynthetic organisms possess a variety of mechanisms to achieve balance between absorbed light (source) and the capacity to metabolically utilize or dissipate this energy (sink). While regulatory processes that detect changes in metabolic status/balance are relatively well-studied in plants, analogous pathways remain poorly characterized in photosynthetic microbes. Herein, we explore systemic changes that result from alterations in carbon availability in the model cyanobacterium Synechococcus elongatus PCC 7942 by taking advantage of an engineered strain where influx/efflux of a central carbon metabolite, sucrose, can be regulated experimentally. We observe that induction of a high-flux sucrose export pathway leads to depletion of internal carbon storage pools (glycogen), and concurrent increases in photosynthetic parameters. Further, a proteome-wide analysis and fluorescence reporter-based analysis revealed that upregulated factors following the activation of the metabolic sink are strongly concentrated on ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) and axillary modules involved in Rubisco maturation. Carboxysome number and Rubisco activity also increase following engagement of sucrose secretion. Conversely, reversing the flux of sucrose by feeding exogenous sucrose heterologously results in increased glycogen pools, decreased Rubisco abundance, decreased photosystem II quantum efficiency, and carboxysome reorganization. Our data suggest that Rubisco activity and organization are key outputs connected to regulatory pathways involved in metabolic balancing in cyanobacteria. One sentence summaryRubisco activity and organization are key outputs involve in source-sink balancing in cyanobacteria.

microbiology↗