Biology subjects

Frokjaer-Jensen, C.

Publications and source records attributed to Frokjaer-Jensen, C..

6 recordsLinked to original sources

bioRxiv2026

Evolution of mechanical chromatin insulators from selfish genetic elements

Genomes require physical boundaries to separate active and repressed chromatin, a function traditionally attributed to insulator proteins. Here, we show that genomic insulation can also emerge from physical properties encoded directly within the DNA polymer. In C. elegans, transcription-coupled mutational bias remodels Helitron transposon minisatellites to match the DNA helical repeat. The resulting 10-bp periodicity (PATCs) encodes intrinsic curvature, creating topologically responsive DNA elements that favor local deformation under supercoiling, disrupt canonical nucleosome organization, and protect germline genes from progressive and heritable silencing. Rather than acting as short protein-binding motifs, Helitron-derived PATCs form extended barrier elements that limit the stabilization of repressive chromatin. Comparative analyses suggest that related sequence-encoded mechanical signatures recur across Metazoa, including at Drosophila insulators, human CTCF sites, and active human LINE-1 retrotransposons. Thus, sequence-encoded mechanics provide an evolutionarily accessible substrate for chromatin insulation--a physical layer of genome organization that can be co-opted by host genomes to protect gene expression, and potentially retained by selfish elements for their own persistence.

molecular biology↗

bioRxiv2025

HP1 dimerization but not LLPS drives the condensation and segregation of H3K9me-marked chromatin

Heterochromatin protein 1 (HP1) is a conserved chromatin-associated factor implicated in the establishment and maintenance of H3K9me-marked heterochromatin, potentially through phase separation-mediated condensation. Whether HP1 promotes heterochromatin condensation primarily through its dimerization or liquid-liquid phase separation (LLPS) remains unresolved. Using the C. elegans HP1 orthologue HPL-2 and a combined in vitro-in vivo approach, we systematically dissected the molecular determinants of HPL-2 function in heterochromatin condensation. Through specific mutants, we demonstrate that HPL-2 dimerization is essential for condensing H3K9me chromatin arrays in vitro and for maintaining HPL-2 and H3K9me heterochromatin foci in C. elegans embryos, while LLPS enhances or stabilizes these foci. We further show that HPL-2 dimerization is sufficient to mediate segregation of H3K9me from unmodified chromatin arrays in vitro, forming spatially distinct H3K9me-enriched condensates. Surprisingly, HPL-2 mutants defective in heterochromatin foci formation cause only minor transcriptional changes among genes associated with H3K9me-marked heterochromatin, implying that HP1-dependent heterochromatin foci and gene silencing are not tightly coupled in vivo. Nonetheless, these mutant C. elegans exhibit profound physiological and developmental defects. Our findings establish dimerization as the principal molecular mechanism of HP1-driven H3K9me-chromatin condensation, elucidate an auxiliary role of LLPS, and reveal the uncoupling between HP1-dependent heterochromatin and transcriptional regulation.

biochemistry↗

bioRxiv2024

MORC-1 is a key component of the C. elegans CSR-1 germline gene licensing mechanism

The Argonaute CSR-1 is essential for germline development in C. elegans. Mutation of csr-1 downregulates thousands of germline-expressed genes, leading to the model that the CSR-1-mediated small RNA pathway promotes, or "licenses," gene expression by an unknown mechanism. CSR-1 also silences a limited number of genes through its canonical endonucleolytic "slicer" activity. We show that the GHKL-type ATPase MORC-1, a CSR-1 slicing target, over-accumulates at CSR-1 "licensed" target genes in csr-1(-), which correlates with ectopic gain of H3K9me3, H3K36me3 loss, and gene downregulation. Loss of morc-1 rescues csr-1(-) defects, while overexpressing MORC-1 in the germline of wild-type worms is sufficient to cause sterility and downregulate CSR-1 targets. These results show that MORC-1 overexpression in csr-1(-) is a primary driver of the CSR-1-mediated gene licensing mechanism. One-Sentence SummaryMORC-1 acts downstream of CSR-1 to regulate germline chromatin states and is a key component of the gene licensing mechanism.

molecular biology↗

bioRxiv2024

In vivo reprogramming of Caenorhabditis elegans leads to heterogeneous effects on lifespan

In the last decade, cellular reprogramming of fully differentiated cells to pluripotent stem cells has become of great interest. Importantly, cellular reprogramming by expression of Oct4, Sox2, Klf4, and cMyc (OSKM) can ameliorate age-associated phenotypes in multiple tissues and extend lifespan in progeroid and aged wild-type mice. Surprisingly, the effects of in vivo reprogramming have not been deeply investigated in any other model organisms. Here, for the first time, we induce in vivo reprogramming in C. elegans using a heat-inducible system at multiple developmental and adult stages. Similar to mice, expression of the reprogramming factors leads to premature death with different levels of toxicity at distinct developmental stages and aging. In vivo reprogramming in C. elegans might represent a valuable tool to improve our understanding of development and in vivo reprogramming.

cell biology↗

bioRxiv2020

A single nucleotide change underlies the genetic assimilation of a plastic trait

Genetic assimilation - the evolutionary process by which an ancestral environmentally sensitive phenotype is made constitutive - is a fundamental concept in biology. Its evolutionary relevance is debated, and our understanding of its prevalence, and underlying genetics and molecular mechanisms, is poor. Matricidal hatching is an extreme form of maternal provisioning induced by adverse conditions, which varies among Caenorhabditis elegans populations. We identified wild isolates, sampled from natural populations across multiple years and locations, that express a derived state of near-constitutive matricidal hatching. A single amino acid change in kcnl-1, encoding a small-conductance calcium-activated potassium channel subunit, explains most of this variation. A gain-of-function mutation altering the S6 transmembrane domain causes inappropriate activation of the K+ channel, leading to reduced vulval muscle excitability, and thus reduced expulsion of embryos, irrespective of environment. Using reciprocal allelic replacements, we show that this amino acid change is sufficient to induce constitutive matricidal hatching whilst re-establishing the ancestral protein abolishes matricidal hatching and restores egg-laying, thereby doubling lifetime reproductive fitness under benign conditions. While highly deleterious in the laboratory, experimental evolution showed that KNCL-1(V530L) is maintained under fluctuating resource availability. Selection on a single point mutation can therefore underlie the genetic assimilation of an ancestrally plastic trait with drastic life-history consequences.

evolutionary biology↗

bioRxiv2019

SapTrap assembly of C. elegans MosSCI transgene vectors

The Mos1-mediated Single-Copy Insertion (MosSCI) method is widely used to establish stable Caenorhabditis elegans transgenic strains. Cloning MosSCI targeting plasmids can be cumbersome because it requires assembling multiple genetic elements including a promoter, a 3UTR and gene fragments. Recently, Schwartz and Jorgensen developed the SapTrap method for the one-step assembly of plasmids containing components of the CRISPR/Cas9 system for C. elegans (Schwartz and Jorgensen 2016 Genetics, 202:1277-1288). Here, we report on the adaptation of the SapTrap method for the efficient and modular assembly of a promoter, 3UTR and either 2 or 3 gene fragments in a MosSCI targeting vector in a single reaction. We generated a toolkit that includes several fluorescent tags, components of the ePDZ/LOV optogenetic system and regulatory elements that control gene expression in the C. elegans germline. As a proof of principle, we generated a collection of strains that fluorescently label the endoplasmic reticulum and mitochondria in the hermaphrodite germline and that enable the light-stimulated recruitment of mitochondria to centrosomes in the one-cell worm embryo. The method described here offers a flexible and efficient method for assembly of custom MosSCI targeting vectors.

genetics↗