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PubMed1987

Transitions, transversions, and the molecular evolutionary clock.

Nucleotide substitutions in the form of transitions (purine-purine or pyrimidine-pyrimidine interchanges) and transversions (purine-pyrimidine interchanges) occur during evolution and may be compiled by aligning the sequences of homologous genes. Referring to the genetic code tables, silent transitions take place in third positions of codons in family boxes and two-codon sets. Silent transversions in third positions occur only in family boxes, except for A = C transversions between AGR and CGR arginine codons (R = A or G). Comparisons of several protein genes have been made, and various subclasses of transitional and transversional nucleotide substitutions have been compiled. Considerable variations occur among the relative proportions of transitions and transversions. Such variations could possibly be caused by mutator genes, favoring either transitions or, conversely, transversions, during DNA replication. At earlier stages of evolutionary divergence, transitions are usually more frequent, but there are exceptions. No indication was found that transversions usually originate from multiple substitutions in transitions.

Animals↗
PubMed1987

Evolution from primordial oligomeric repeats to modern coding sequences.

It seems as though nature was most innovative at the very beginning of life on this Earth a few billion years ago. For example, the functional competence of most, if not all, of the sugar-metabolizing enzymes was clearly established before the division of eukaryotes from prokaryotes eons ago, each critical active-site amino acid sequence being conserved ever since by bacteria as well as by mammals. I contend that this initial innovativeness was due to the first set of coding sequences being repeats of base oligomers, thus encoding polypeptide chains of various periodicities; such periodical polypeptide chains can easily acquire alpha-helical and beta-sheet-forming segments. In fact, the entire length of sugar-metabolizing enzymes is comprised of alternating alpha-helical and beta-sheet-forming segments. In the prebiotic (therefore nonenzymatic) replication of nucleic acids, what was in short supply was long templates, for there apparently was no inherent obstacle in copying of long templates, if such existed, in the presence of Zn2+. I submit that in this prebiotic condition, only those nucleotide oligomers that were internal doubles were automatically assured of progressive elongation to become long templates. For example, a decamer that was a pentameric repeat and its complementary sequence may pair unequally to initiate the next round of replication: first unit pairing with second, and a paired segment serving as a primer. As a consequence of this unequal pairing, decameric templates managed to become pentadecameric templates only after one round of replication, and this elongation process had no inherent limit.

Amino Acid Sequence↗
PubMed1992

The formation of internal 6-methyladenine residues in eucaryotic messenger RNA.

1. The formation of internal 6-methyladenine (m6A) residues in eucaryotic messenger RNA (mRNA) is a postsynthetic modification in which S-adenosyl-L-methionine (SAM) serves as the methyl donor. 2. Of the methyl groups incorporated into mature mRNA 30-50% occur in m6A residues. 3. Although most cellular and certain viral mRNAs contain at least one m6A residue, some transcripts such as those coding for histone and globin are completely lacking in this modification. 4. 6-Methyladenine residues have also been localized to heterogeneous nuclear RNA (HnRNA), and for the most part these residues are conserved during mRNA processing. 5. In all known cases, the m6A residues are also found in a strict consensus sequence, Gm6AC or Am6AC, within the transcript. 6. Although the biological significance of internal adenine methylation in eucaryotic mRNA remains unclear, a great deal of research has indicated that this modification may be required for mRNA transport to the cytoplasm, the selection of splice sites or other RNA processing reactions.

Adenine↗
PubMed2025

Analog epigenetic memory revealed by targeted chromatin editing.

Cells store information by means of chromatin modifications that persist through cell divisions and can hold gene expression silenced over generations. However, how these modifications may maintain other gene expression states has remained unclear. This study shows that chromatin modifications can maintain a wide range of gene expression levels over time, thus uncovering analog epigenetic memory. By engineering a genomic reporter and epigenetic effectors, we tracked the gene expression dynamics following targeted perturbations to the chromatin state. We found that distinct grades of DNA methylation led to corresponding, persistent gene expression levels. Altering the DNA methylation grade, in turn, resulted in permanent loss of gene expression memory. Consistent with experiments, our chromatin modification model indicates that analog memory arises when the positive feedback between DNA methylation and repressive histone modifications is lacking. This discovery will lead to a deeper understanding of epigenetic memory and to new tools for synthetic biology.

Epigenesis, Genetic↗
PubMed2026

Genome-wide profiling of histone modifications and transcription factor binding at single-cell resolution by DeChIC-seq.

Mapping of protein-DNA interactions at single-cell resolution remains a central challenge in epigenomics, particularly for transcription factors (TFs), whose sparse binding limits reliable detection. Here, we establish DeChIC-seq (DNA Deaminase-based Chromatin Immuno-Conversion sequencing), a conversion-based strategy that uses a protein A-DddAtox fusion to directly record protein-DNA interactions by inducing localized C-to-U conversions near antibody-bound chromatin. Retaining genome-wide background sequence information without immunoprecipitation, DeChIC-seq enables profiling of histone modifications and sensitive detection of TF binding. Integration with single-cell whole-genome amplification extends DeChIC-seq to single-cell applications (scDeChIC-seq), enabling chromatin profiling of individual cells. Applied to mouse embryogenesis, scDeChIC-seq resolves lineage-specific chromatin states through profiling of H3K4me3, CTCF, and RAD21 and sensitively detects TF binding, including that of NR5A2, TFAP2C, and KLF5, from extremely limited blastomere inputs. This underscores its strong potential for detecting TF-binding sites in scarce biological samples. DeChIC-seq establishes a conversion-based framework for chromatin profiling that enables mechanistic dissection of TF-driven gene regulation across rare cells, developmental systems, and disease contexts.

Animals↗
PubMed2026

Improved spike-in normalization clarifies the relationship between active histone modifications and transcription.

Spike-in normalization enables quantitative analysis of chromatin immunoprecipitation sequencing (ChIP-seq) signal. Here we introduce a robust dual spike-in normalization approach for ChIP-seq (ChIP-wrangler), optimize parameters and verify its accuracy in quantifying changes in ChIP-seq signal and detecting technical artifacts. We use ChIP-wrangler to revisit recent claims that active histone marks depend on transcription. We show that acute depletion of RNA polymerase II (RNAPII) has a modest impact on H3K27ac levels, with only 6% of peaks significantly changing after RNAPII depletion, indicating that histone acetylation maintenance is not entirely dependent on ongoing transcription. Promoters and enhancers are differentially affected, with 82% of decreasing acetylation peaks located at promoter-distal elements with enhancer-related motifs. ChIP-wrangler provides increased rigor and 'guardrails' for successful spike-in normalization and, as applied here, refines the understanding of crosstalk between RNAPII activity and transcription-associated histone marks.

Histones↗
PubMed2025

Single-cell multi-omic detection of DNA methylation and histone modifications reconstructs the dynamics of epigenomic maintenance.

DNA methylation and histone modifications encode epigenetic information. Recently, major progress was made to measure either mark at a single-cell resolution; however, a method for simultaneous detection is lacking, preventing study of their interactions. Here, to bridge this gap, we developed scEpi2-seq. Our technique provides a readout of histone modifications and DNA methylation at the single-cell and single-molecule level. Application in a cell line with the FUCCI cell cycle reporter system reveals how DNA methylation maintenance is influenced by the local chromatin context. In addition, profiling of H3K27me3 and DNA methylation in the mouse intestine yields insights into epigenetic interactions during cell type specification. Differentially methylated regions also demonstrated independent cell-type regulation in addition to H3K27me3 regulation, which reinforces that CpG methylation acts as an additional layer of control in facultative heterochromatin.

DNA Methylation↗