Biology subjects

Gladyshev, E.

Publications and source records attributed to Gladyshev, E..

7 recordsLinked to original sources

bioRxiv2026

SAD-6/ATRX enables broad genome surveillance and defense in fungi

The chromatin remodeler ATRX and its orthologs maintain genome function by regulating repetitive DNA and dynamic chromatin, and their activities have been canonically associated with replication-independent deposition of the histone H3.3 variant. This model is difficult to reconcile with fungi, which encode ATRX orthologs but lack H3 variants that may separately support replication-coupled and replication-independent deposition. Here we show that the fungal ATRX ortholog SAD-6 instead relies on a highly divergent histone H4 variant (H4v) to mediate broad genome surveillance and defense. Deposition of H4v is strictly SAD-6-dependent and thus provides a sensitive genome-wide readout of SAD-6 activity, revealing its functions at telomeres, tRNA and rDNA loci, AT-rich DNA, artificial transgenes, decaying mobile elements, and many genic regions. We further show that SAD-6 is required for a pathway of repeat-induced point mutation (RIP) that also requires DIM-5, a conserved SUV39 methyltransferase that mediates trimethylation of histone H3 lysine-9 in heterochromatin. Together, these findings establish ATRX-like remodelers as broad regulators of genome surveillance and defense in fungi that act through a highly divergent histone H4 variant rather than H3.3. Given that RIP is proposed to recognize repetitive DNA via recombination-independent homologous pairing, the requirement for SAD-6 in RIP suggests that ATRX-like remodelers may couple DNA pairing to heterochromatin nucleation on repeats.

molecular biology↗

bioRxiv2024

Properties of DNA in concentrated aqueous solutions of LiCl suggest transition to C-DNA

C-DNA represents a canonical DNA form related to B-DNA. While C-DNA is known to exist in air-dried fibers, its occurrence in aqueous solutions remains subject to debate. In fibers, the transition from B- to C-DNA is promoted by reduced hydration in the presence of certain monovalent cations (most notably Li+), and this process is generally associated with an increase in the helical twist. To understand if the B-to-C transition can occur in liquid media in principle, we analyzed properties of several circular DNA substrates in aqueous solutions with varying concentrations of LiCl (0.1-8 M). To this end, we estimated changes in the helical twist directly from circular dichroism (CD) spectra of a large supercoiled plasmid and, in parallel, assayed conformational changes in DNA minicircles by atomic force microscopy (AFM). We found that the helical twist increased continuously over the entire range of tested LiCl concentrations, without being subject to saturation even at 8 M LiCl. The overall increase in the helical twist was compatible with the B-to-C transition occurring in solution at a concentration of LiCl of about 8 M, suggesting that C-DNA should be stable above this level.

biophysics↗

bioRxiv2024

Chromatin remodeling initiates transcriptional and post-transcriptional silencing in Neurospora

In eukaryotes, repetitive DNA can become silenced de novo, either transcriptionally or post-transcriptionally, by processes independent of strong sequence-specific cues. The mechanistic nature of such processes remains poorly understood. We found that in the fungus Neurospora crassa, de novo initiation of both transcriptional and post-transcriptional silencing was linked to perturbed chromatin, which was produced experimentally by the aberrant activity of transcription factors at the tetO operator array. Transcriptional silencing was mediated by canonical constitutive heterochromatin. On the other hand, post-transcriptional silencing resembled repeat-induced quelling but occurred normally when homologous recombination was inactivated. All silencing of the tetO array was dependent on SAD-6 (a fungal ortholog of the SWI/SNF chromatin remodeler ATRX), which was required to maintain nucleosome occupancy at the perturbed locus. In addition, we found that two other types of sequences (the lacO array and native AT-rich DNA) could also undergo recombination-independent quelling associated with perturbed chromatin. These results suggested a model in which the de novo initiation of transcriptional and post-transcriptional silencing is coupled to the remodeling of perturbed chromatin. SIGNIFICANCE STATEMENTThis study addresses an enigmatic question of how transcriptional and post-transcriptional gene silencing can be initiated de novo in the absence of strong sequence-specific cues. Using the fungus Neurospora crassa as a model organism, we found that both types of silencing can be triggered in mitotic cells by the remodeling of a transiently perturbed (nucleosome-depleted) chromatin state. In this system, the initiation of silencing requires SAD-6, a conserved SWI/SNF chromatin remodeler orthologous to ATRX that has been already implicated in repetitive DNA silencing in fungi, plants, and animals. Thus, the model proposed in this study may underpin a range of gene-silencing phenomena observed in other eukaryotes.

molecular biology↗

bioRxiv2021

Regulation of DIM-2-dependent repeat-induced point mutation (RIP) by the recombination-independent homologous DNA pairing in Neurospora crassa

Repeat-induced point mutation (RIP) is a genetic process that creates cytosine-to-thymine (C-to-T) transitions in duplicated genomic sequences in fungi. RIP detects duplications irrespective of their origin, particular sequence, coding capacity, or genomic positions. Previous studies suggested that RIP involves a cardinally new mechanism of sequence recognition that operates on intact double-stranded DNAs. In the fungus Neurospora crassa, RIP can be mediated by a putative C5-cytosine methyltransferase (CMT) RID or/and a canonical CMT DIM-2. These distinct RIP pathways feature opposite substrate preferences: RID-dependent RIP is largely limited to the duplicated sequences, whereas DIM-2-dependent RIP preferentially mutates adjacent non-repetitive regions. Using DIM-2-dependent RIP as a principal readout of repeat recognition, here we show that GC-rich repeats promote stronger RIP compared to AT-rich repeats (independently of their intrinsic propensities to become mutated), with the relative contribution of AT base-pairs being close to zero. We also show that direct repeats promote much more efficient DIM-2-dependent RIP than inverted repeats; both the spacer DNA between the repeat units (the linker) and the flanking regions are similarly affected by this process. These and other results support the idea that repeat recognition for RIP involves formation of many short interspersed quadruplexes between homologous double-stranded DNAs, which need to undergo concomitant changes in their linking number to accommodate pairing. SUMMARYDuring repeat-induced point mutation (RIP) gene-sized duplications of genomic DNA are detected by a mechanism that likely involves direct pairing of homologous double-stranded DNAs. We show that DIM-2-dependent RIP, triggered by closely-positioned duplications, is strongly affected by their relative orientations (direct versus inverted). We also show that GC-rich repeats promote RIP more effectively than AT-rich repeats. These results support a model in which homologous dsDNAs can pair by establishing interspersed quadruplex-based contacts with concomitant changes in their supercoiling status.

genetics↗

bioRxiv2019

Recombination-independent recognition of DNA homology for meiotic silencing in Neurospora crassa

Pairing of homologous chromosomes represents a critical step of meiosis in nearly all sexually reproducing species. While in some organisms meiotic pairing requires programmed DNA breakage and recombination, in many others it engages homologous chromosomes that remain apparently intact. The mechanistic nature of such recombination-independent pairing represents a fundamental question in molecular genetics. Using meiotic silencing by unpaired DNA (MSUD) in Neurospora crassa as a model process, we demonstrate the existence of a cardinally different approach to DNA homology recognition in meiosis. The main advantage of MSUD over other experimental systems lies in its ability to identify any relatively short DNA fragment lacking a homologous allelic partner. Here we show that MSUD does not rely on the canonical mechanism of meiotic recombination, yet it is promoted by REC8, a conserved component of the meiotic cohesin complex. We also show that certain patterns of interspersed homology are recognized as pairable during MSUD. Such patterns need to be co-linear and must contain short tracts of sequence identity spaced apart with a periodicity of 21 or 22 base-pairs. By using these values as a guiding parameter in all-atom molecular modeling, we discover that homologous double-stranded DNA molecules can associate by forming quadruplex-based contacts with an interval of 2.5 helical turns, which requires right-handed plectonemic coiling and additional conformational changes in the intervening double-helical segments. These results (i) reconcile genetic and biophysical lines of evidence for the existence of direct homologous dsDNA-dsDNA pairing, (ii) identify a role for this process in initiating post-transcriptional silencing, and (iii) suggest that chromosomes are cross-matched in meiosis by a precise mechanism that operates on intact double-stranded DNA molecules.

molecular biology↗

bioRxiv2018

Structural Aspects of Homologous DNA-DNA Interactions Revealed by Partitioning of RIP Mutations

In some fungi, a process known as Repeat-Induced Point mutation (RIP) can accurately identify and mutate nearly all genesized DNA repeats present in the haploid premeiotic nuclei. Studies of RIP in Neurospora crassa have suggested that the sequence homology is detected between intact double helices without strand separation and participation of RecA homologs. These studies relied on the aggregated number of mutations as a simple quantitative readout of RIP activity and did not try interpret the distributions of mutations along DNA. Important additional information can be extracted by transforming these distributions into profiles of a new parameter called partitioned RIP propensity (PRP) which takes into account the site density as well as the sequence context. This approach revealed surprising systematic variations of PRP due to the position of a given DNA segment relative to the homology boundaries and its topology. Notably, identical pairs of direct versus inverted repeats produce very distinct PRP profiles. This effect could be rationalized assuming a specific redistribution of the supercoiling stress produced by the previously discovered untwisting of paired of DNA homologs. Similar mechanisms account for other persistent features of PRP profiles, and this general topological model raises an intriguing possibility that local DNA supercoiling provoked by homologous dsDNA-dsDNA pairing can modulate the overall structure and properties of repetitive DNA. These effects can be particularly strong in the context of long tandem repeat arrays that are typically present at the (peri)centromeric regions of chromosomes.

biophysics↗

bioRxiv2017

DNA sequence homology induces cytosine-to-thymine mutation by a heterochromatin-related pathway

In the genomes of many eukaryotes, including mammals, highly repetitive DNA is normally associated with histone H3 lysine-9 di-/trimethylation (H3K9me2/3) and C5-cytosine methylation (5mC) in the context of heterochromatin. In the fungus Neurospora crassa, H3K9me3 and 5mC are catalyzed, respectively, by a conserved SUV39 histone methylase DIM-5 and a DNMT1-like cytosine methylase DIM-2. Here we show that DIM-2 can also mediate cytosine-to-thymine mutation of repetitive DNA during the pre-meiotic process known as Repeat-Induced Point mutation (RIP) in N. crassa. We further show that DIM-2-dependent RIP requires DIM-5, HP1, and other heterochromatin factors, implying the involvement of a repeat-induced heterochromatin-related process. Our previous findings suggest that the mechanism of homologous repeat recognition for RIP involves direct pairwise interactions between co-aligned double-stranded (ds) DNA segments. Our current findings therefore raise the possibility that such pairing interactions may occur not only in pre-meiotic but also in vegetative cells, where they may direct heterochromatin assembly on repetitive DNA. In accord with this possibility, we find that, in vegetative cells of N. crassa, our model repeat array comprising only four 674-bp sequence copies can trigger a low level of DIM-5-dependent 5meC. We thus propose that homologous dsDNA/dsDNA interactions between a small number of repeat copies can nucleate a transient state of heterochromatin and that, on longer repeat arrays, such interactions lead to the formation of stable heterochromatin. Since the number of possible pairwise dsDNA/dsDNA interactions will scale non-linearly with the number of repeats, this mechanism provides an attractive way of creating the extended domains of constitutive heterochromatin found in pericentromeric and subtelomeric regions.

molecular biology↗