Biology subjects

Barik, S.

Publications and source records attributed to Barik, S..

13 recordsLinked to original sources

bioRxiv2026

Sequence-Dependent DNA Base Selection Fidelity: A Kinetics-based Model and its validation

DNA replication achieves error rates as low as 10-9-10-11 per base pair through the combined contribution of base selection, exonuclease proofreading, and mismatch repair. Among those processes, base selection determines the initial level of accuracy and is strongly modulated by local sequence context. Existing models address this dependence either by fitting individual rate constants for each sequence context or by invoking the global template properties, neither of which derives sequence dependence from the underlying thermodynamics and kinetics of base pair formation. Here we present a mechanism for sequence-dependent base selection fidelity, built from two physical properties: nearest-neighbor stacking thermodynamics and directional kinetic asymmetry. The model fits the experimentally observed mutation spectra from three mismatch repair-deficient organisms well (r=0.74, 0.70, and 0.63), and predicts that base-selection accuracy varies non-monotonically with temperature in a sequence-dependent manner. Our model, therefore, provides a framework that connects sequence-dependent thermodynamic and kinetic effects during nucleotide incorporation to experimentally observed mutation rates.

biochemistry↗

bioRxiv2026

Directional purine pyrimidine asymmetry in CRISPR repeats facilitates spacer acquisition

Spacer acquisition is the primary and essential step of CRISPR-Cas adaptive immunity in most prokaryotes and occurs preferentially at the leader-repeat junction of the CRISPR array. Despite the conservation of the adaptation machinery, spacer acquisition efficiencies and site-specificity vary markedly across CRISPR-Cas systems, suggesting that the local sequence architecture of the repeat and leader-repeat junction may contribute to the variation in the adaptation efficiency. To investigate this possibility, we systematically analyzed the direct repeat sequences and the terminal base pairs at the 3' end of leader regions adjacent to the integration site. We identify a conserved asymmetric purine-pyrimidine (RY) distribution within direct repeats, characterized by a 5' pyrimidine-rich half and a 3' purine-rich half, together with purine enrichment at the 3' end of leader sequences. Building on these observations, we develop a mechanistic model for spacer acquisition by demonstrating the importance of the sequence motif of the first direct repeat and the leader-repeat junction, using the sequence-dependent asymmetric cooperativity model that captures DNA unzipping kinetics. According to our model, sequences with such directional RY-asymmetric nucleotide distribution as direct repeats have high unzipping propensity and thus can promote efficient spacer integration. Consistent with this mechanism, experimental data from previous studies show that repeats with stronger asymmetry and appropriately positioned pyrimidine-to-purine transition sites are associated with larger spacer counts within CRISPR arrays. These findings reveal a conserved architectural feature of CRISPR repeats and provide a mechanistic model connecting DNA sequence organization to efficiency.

genomics↗

bioRxiv2026

Curcumin - Gold Nanocomposites for Enhanced Doxorubicin Delivery: Molecular Mechanisms of Loading and Membrane Interactions

Curcumin-functionalized gold nanoclusters are promising platforms for catalysis and drug delivery, however, the molecular determinants governing their stability, morphology, and solvent response remain poorly understood. Here, microsecond all-atom molecular dynamics simulations were employed to investigate a 2 nm gold nanoparticle noncovalently coated with different curcumin forms, including the neutral enol and trans-keto tautomers, the deprotonated enolate, and their mixtures in water-ethanol and water-methanol solvents. Region-resolved analyses of the radius of gyration, density profiles, and surface coverage reveal that neutral enol and trans forms generate compact assemblies with near-complete surface coverage, whereas enolate-rich systems adopt more expanded conformations with greater solvent exposure. Mixed systems preserve these intrinsic packing characteristics while improving overall surface coverage. Solvent substitution from ethanol to methanol reduces {pi}-{pi} stacking, strengthens Au-curcumin interactions, and increases surface coverage, yielding more compact nanostructures. Free energy and potential of mean force calculations further indicate that deprotonated curcumin most effectively screens Au-Au interactions and promotes nanoparticle dispersion, whereas the neutral tautomers provide moderate stabilization. Curcumin functionalization also enhances the loading of the anticancer drugs doxorubicin (DOX) and niraparib (NIR) onto Au nanoparticles. Membrane interaction with AuDOX/NIR-Curcumin simulations show that Enolate(An)-containing systems form more extended structures and interact more weakly with both negatively charged and neutral DMPC membranes. In contrast, neutral curcumin complexes form compact, positively charged assemblies with stronger interactions with the negatively charged model membrane. Furthermore, these interactions are predominantly driven by electrostatic attraction and are substantially weaker with the neutral DMPC membrane. Overall, these findings demonstrate how curcumin tautomeric state and solvent environment cooperatively govern interfacial organization and colloidal stability, drug-loading, and membrane interactions. This provides molecular-level design principles for curcumin-based gold nanocarriers for catalysis, sensing, and drug delivery applications.

biophysics↗

bioRxiv2026

Ineffectual Genomic Error Correction Under Environmental Perturbation Dynamically Regulates Mutational Supply and Robustness

Adaptive evolution depends on the supply of heritable variation, yet excessive mutation threatens viability by degrading essential molecular functions. Here, we show that this trade-off emerges naturally from the kinetic proofreading mechanism that controls replication fidelity. In our model, environmental shifts alter the optimal driving rate constant of proofreading enzymes, transiently elevating replication error rates and triggering rapid evolutionary change until a new fidelity optimum is reached. This produces alternating periods of stasis and rapid adaptation, consistent with punctuated equilibrium. We further show that coding-region length and population size jointly determine whether adaptation succeeds or mutational collapse occurs, reflecting the balance between mutation supply and error tolerance predicted by the drift-barrier principle. These results provide a molecular mechanistic basis for nearly neutral evolutionary dynamics and illustrate how genomic organization constrains long-term evolutionary resilience.

evolutionary biology↗

bioRxiv2026

Candida albicans Snf2 modulates the response to DNA damage by regulating gene expression and uptake of the genotoxic stressors

The SWI/SNF complex comprising of the catalytic subunit, Snf2, is a key regulator of gene expression and DNA damage repair in eukaryotic cell. Candida albicans Snf2 is known to regulate hyphal formation. In this paper, we have investigated the role of this protein in DNA damage response. We show that CaSnf2 is required for cell division as deletion of both copies of SNF2 leads to increased duplication time. The mutant cells form clumps with increased chitin and {beta}-glucan deposition on the cell wall. The altered cell wall phenotype leads to reduced uptake of genotoxic stressors leading to increased resistance to both methyl methane sulfonate (MMS) and hydroxyurea (HU). In addition, resistance of Casnf2{Delta} cells to MMS also appears to be mediated by upregulation of CaRAD9 expression by CaFun30, an ATP-dependent chromatin remodeling protein, and CaRtt109, a fungal-specific histone acetyltransferase. The response of Casnf2{Delta} to genotoxic stressors is at variance with the response of Scsnf2{Delta} mutant, highlighting the differences in DNA damage response/repair pathway between the two organisms. Finally, we show that Casnf2{Delta} mutants are extremely sensitive to azoles due to downregulation of multi-drug resistance pumps leading to reduced efflux of the drug.

microbiology↗

bioRxiv2025

Genome size and nucleotide skews as predictors of bacterial growth rate

Bacterial growth rates are constrained by genome replication, yet the role of replication kinetics in bacterial growth rates remains incompletely understood. Here, we examine if genome size, replichore organization, and nucleotide compositional asymmetry are reasonable predictors of bacterial doubling times. In free-living bacteria, both genome size and the length of the longest replichore are found to correlate positively with doubling time, pointing to an influence of replication dynamics on bacterial growth rates. Moreover, fast-growing bacteria are shown to exhibit stronger nucleotide compositional skew. Incorporating skew into the model substantially improves predictive accuracy, suggesting that compositional asymmetry in genomes may facilitate replication fork progression and thereby enhance growth rates. Based on these observations, we speculate that nucleotide skew may play a potential adaptive role in bacterial genome replication. To assess whether the observed association between genome architecture and growth rate reflects an evolutionary signature or a mechanistic link, we reconstructed ancestral states and found that the model fits ancestral traits more strongly, with predictive strength (R2) decreasing progressively along the evolutionary tree as successive speciations occur. We speculate that this association has been stronger early in bacterial evolution and became subsequently screened as organisms diversified and increased in ecological and physiological complexity.

genomics↗

bioRxiv2025

Replication rate-information storage trade-off shapes genome architecture across domains

Genome size varies widely among organisms, from compact genomes of bacteria to vast and complex genomes of eukaryotes. In this study, we theoretically identify the evolutionary pressures that may have driven this divergence in genome size. We use a parameter-free model to study genome size evolution under selection pressure to minimize replication time and maximize information storage capacity. We substantiate this choice by demonstrating a correlation between replication time and genome size using literature data. We show that bacteria tend to reduce genome size, constrained by a single replication origin, while eukaryotes expand their genomes by incorporating multiple replication origins. We propose a connection between genome size and cellular energetics, suggesting that endosymbiotic organelles, mitochondria and chloroplasts, evolutionarily regulate the number of replication origins, thereby influencing genome size in eukaryotes. We substantiate this claim by showing a correlation between organelle count and genome size data. We argue that nucleotide skews, present in nearly all genomes studied, and are used to identify replication origins, directly influence DNA unzipping kinetics, and hence its replication time. This connection enables us to derive nearly universal observations, such as Chargaffs second parity rule and replichore symmetrization, as adaptive consequences. We argue that high skews lead to faster replication, and substantiate it by showing a correlation between skews and replication speed in bacteria. The model reproduces more evolutionary genomic observations, such as a general preference for deletions over insertions across domains, and elongation and high variance of genome size under reduced selection pressure for replication rate, an integral component of C-value paradox. We highlight the possibility of regulation of the firing of latent replication origins in response to cues from the extracellular environment, leading to the regulation of cell cycle rates in multicellular eukaryotes. Significance StatementUnderstanding the forces shaping genome architecture is a long-standing challenge in evolutionary biology. Our study demonstrates that the balance between replication speed, influenced by nucleotide skews, and information storage, constrained by cellular energetics, drives the divergence in genome size between bacteria and eukaryotes. By quantifying selection pressure as the ratio of replication time to genomic information storage capacity, we show that this pressure enforces adaptive constraints, giving rise to observed features such as symmetric replichores and Chargaffs second parity rule. These insights not only help us resolve an enduring evolutionary puzzle, but also offer a unified framework linking genome organization, cellular specialization, and even potential mechanisms underlying carcinogenesis.

genomics↗

bioRxiv2025

Non-enzymatic error correction in self-replicators without extraneous energy supply

Accurate propagation of sequence information in nucleic acids is central to the evolutionary dynamics of self-replicating systems. Modern biological systems achieve high fidelity using enzymes that actively correct errors through energy-driven mechanisms. However, such complex machinery was absent under prebiotic conditions. Here, we present a theoretical model of error correction in self-replicating heteropolymers that requires neither enzymes nor an extraneous energy supply. The model relies solely on the free-energy gradient driving strand growth and requires asymmetric cooperativity -- a kinetic asymmetry known to promote unidirectional elongation. Despite its simplicity, we demonstrate that this minimal model facilitates kinetic discrimination between correct and incorrect base pair incorporations, and for specific set of parameters, reproduces the error ratio of [~] 10-4, experimentally observed in passive base selection processes. It replicates key features observed in DNA error correction, including stalling, fraying, next-nucleotide effects, and the speed-accuracy trade-off. Our results provide plausible answers for longstanding questions, such as the energy source for the enhanced base selectivity of passive DNA polymerases and the role of thermodynamics and kinetics of phosphodiester bond formation in error correction. We show that catalysis of the phosphodiester bond plays a central role in error correction, even without explicit enzymatic structural discrimination. This observation points to a plausible pathway for accurate oligomer synthesis under prebiotic conditions, driven solely by the thermodynamic gradient favoring strand elongation. More broadly, the model highlights how persistent molecular order can emerge from non-equilibrium dynamics - a central requirement for emergence of life.

biochemistry↗

bioRxiv2025

Eukaryotes evade information storage-replication rate trade-off with endosymbiont assistance leading to larger genomes

Genome length varies widely among organisms, from compact genomes of prokaryotes to vast and complex genomes of eukaryotes. In this study, we theoretically identify the evolutionary pressures that may have driven this divergence in genome length. We use a parameter-free model to study genome length evolution under selection pressure to minimize replication time and maximize information storage capacity. We show that prokaryotes tend to reduce genome length, constrained by a single replication origin, while eukaryotes expand their genomes by incorporating multiple replication origins. We propose a connection between genome length and cellular energetics, suggesting that endosymbiotic organelles, mitochondria and chloroplasts, evolutionarily regulate the number of replication origins, thereby influencing genome length in eukaryotes. We show that the above two selection pressures also lead to strict equalization of the number of purines and their corresponding base-pairing pyrimidines within a single DNA strand, known as Chagraffs second parity rule, a hitherto unexplained observation in genomes of nearly all known species. This arises from the symmetrization of replichore length, another observation that has been shown to hold across species, which our model reproduces. The model also reproduces other experimentally observed phenomena, such as a general preference for deletions over insertions, and elongation and high variance of genome lengths under reduced selection pressure for replication rate, termed the C-value paradox. We highlight the possibility of regulation of the firing of latent replication origins in response to cues from the extracellular environment leading to the regulation of cell cycle rates in multicellular eukaryotes. Significance StatementUnderstanding the forces shaping genome architecture is a long-standing challenge in evolutionary biology. Our study demonstrates that the balance between replication speed and information storage, constrained by cellular en-ergetics, drives the divergence in genome lengths between prokaryotes and eukaryotes. By quantifying selection pressure as the ratio of replication time to genomic information content, we show that this pressure enforces adaptive constraints, giving rise to features such as symmetric replichores and Chargaffs second parity rule. These insights not only help us resolve an enduring evolutionary puzzle, but also offer a unified framework linking genome organization, cellular specialization, and even potential mechanisms underlying carcinogenesis.

evolutionary biology↗

bioRxiv2021

RTT109 AND Fun30 proteins mediate epigenetic regulation of the DNA damage response pathway in C. albicans

Fun30, an ATP-dependent chromatin remodeller, from S. cerevisiae mediates both regulation of gene expression as well as DNA damage response/repair. In this paper, we have characterized the biochemical and physiological function of Fun30 from the opportunistic fungi, C. albicans. Biochemically, the protein shows DNA-stimulated ATPase activity. Physiologically, the protein co-regulates transcription of RTT109, TEL1, MEC1, and SNF2-genes that encode for proteins involved in DNA damage response and repair pathway. The expression of FUN30, in turn, is regulated by histone H3 acetylation catalysed by Rtt109 encoded by RTT109. The RTT109Hz/FUN30Hz mutant strain shows sensitivity to oxidative stress and resistance to MMS as compared to the wild type strain. Quantitative PCR showed that the sensitivity to oxidative stress results from downregulation of MEC1, RAD9, MRC1 and RAD5 expression; ChIP experiments showed Fun30 but not H3ac regulates the expression of these genes in response to oxidative stress. In contrast, on treatment with MMS, the expression of RAD9 is upregulated and this upregulation is co-regulated by both Fun30 and H3 acetylation catalysed by Rtt109. Thus, Fun30 and H3 acetylation mediate the response of the fungal cell to genotoxic agents in C. albicans by regulating the expression of DNA damage response and repair pathway genes.

molecular biology↗

bioRxiv2020

Unique mutational changes in SARS-CoV2 genome of different state of India

COVID-19 is a global pandemic causing more than 8 million deaths till mid-August, 2020. In India, more than 3 million confirmed cases have been reported although with relatively low death rate of 1.8%. In this study, we sequenced 47 genomes of SARS-CoV-2 from the patients of 13 districts of Uttar Pradesh (UP), the largest state of India using third-generation sequencing technique. The phylogenetic clustering revealed that no UP sample was aligned with the previously defined USA clade, where the mortality was high. We identified 56 distinct SNP variations in the genomes of UP resulting in a unique mutation rate of 1.19% per sequence, which is greater than the value 0.88% obtained for the rest of India. The relatively less death rate in UP indicates that the mutation in the virus is deleterious. Further investigation is required with larger sample size to determine the degree of virulence vis-a-vis SNP variation.

bioinformatics↗

bioRxiv2020

Regulation of NF-kappa B and cell death by bacterial gingipains

The role of non-caspase proteases in the regulation of programmed cell death is relatively under-studied. In this short communication, we report that infection of human gingival fibroblast (HGF) cells ex vivo by Porphyromonas gingivalis rapidly activated NF-kappa B, an antiapoptotic player; PI3 kinase was also induced, and together, they inhibited apoptosis. Later, pro-apoptotic cellular genes, including caspases, were induced. Apoptosis was evidenced by the degradation of nuclear DNA and activation of caspases. Unexpectedly, a P. gingivalis mutant that lacked all three major gingipain proteases was defective in promoting apoptosis, which was consistent with the cleavage of procaspase-3 to the active form by the culture supernatant of wild type P. gingivalis, but not that of the triple protease mutant. These results suggest that cellular death in the P. gingivalis-infected gum may be orchestrated by gingipain-regulated apoptotic factors.

microbiology↗

bioRxiv2016

Viral Quasispecies Reconstruction via Correlation Clustering

RNA viruses are characterized by high mutation rates that give rise to populations of closely related viral genomes, the so-called viral quasispecies. The underlying genetic heterogeneity occurring as a result of natural mutation-selection process enables the virus to adapt and proliferate in face of changing conditions over the course of an infection. Determining genetic diversity (i.e., inferring viral haplotypes and their proportions in the population) of an RNA virus is essential for the understanding of its origin and mutation patterns, and the development of effective drug treatments. In this paper we present QSdpR, a novel correlation clustering formulation of the quasispecies reconstruction problem which relies on semidefinite programming to accurately estimate the sub-species and their frequencies in a mixed population. Extensive comparisons with existing methods are presented on both synthetic and real data, demonstrating efficacy and superior performance of QSdpR.

bioinformatics↗