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Exposure of human cells to interferon-γ (IFNγ) results in a mitotically heritable yet reversible state called long-term transcriptional memory. We previously identified the clustered GBP genes as strongly primed by IFNγ. Here, we discovered that in primed cells, both interferon-responsive transcription factors STAT1 and IRF1 target chromatin with accelerated kinetics upon re-exposure to IFNγ, specifically at promotors of primed genes. Priming does not alter the degree of IFNγ-induced STAT1 activation or nuclear import, indicating that memory does not alter upstream JAK-STAT signaling. We found STAT1 to be critical to establish transcriptional memory but in a manner that is independent of mere transcription activation. Interestingly, while Serine 727 phosphorylation of STAT1 was maintained during the primed state, STAT1 is not required for the heritability of GBP gene memory. Our results suggest that the memory of interferon exposure constitutes a STAT1-mediated, heritable state that is established during priming. This renders GBP genes poised for subsequent STAT1 and IRF1 binding and accelerated gene activation upon a secondary interferon exposure., (© 2023 The Authors. Published under the terms of the CC BY 4.0 license.)

Cytokine activation of cells induces gene networks involved in inflammation and immunity. Transient gene activation can have a lasting effect even in the absence of ongoing transcription, known as long-term transcriptional memory. Here we explore the nature of the establishment and maintenance of interferon γ (IFNγ)-induced priming of human cells. We find that, although ongoing transcription and local chromatin signatures are short-lived, the IFNγ-primed state stably propagates through at least 14 cell division cycles. Single-cell analysis reveals that memory is manifested by an increased probability of primed cells to engage in target gene expression, correlating with the strength of initial gene activation. Further, we find that strongly memorized genes tend to reside in genomic clusters and that long-term memory of these genes is locally restricted by cohesin. We define the duration, stochastic nature, and molecular mechanisms of IFNγ-induced transcriptional memory, relevant to understanding enhanced innate immune signaling., Competing Interests: Declaration of Interests The authors declare no competing interests., (Copyright © 2020 The Authors. Published by Elsevier Inc. All rights reserved.)

Specificity engineering is challenging and particularly difficult for enzymes that have the catalytic machinery and specificity determinants in close proximity. Restriction endonucleases have been used as a paradigm for protein engineering, but successful cases are rare. Here, we present the results of a directed evolution approach to the engineering of a dimeric, blunt end cutting restriction enzyme NlaIV (GGN/NCC). Based on the remote similarity to EcoRV endonuclease, regions for random mutagenesis and in vitro evolution were chosen. The obtained variants cleaved target sites with an up to 100-fold k cat /K M preference for AT or TA (GGW/WCC) over GC or CG (GGS/SCC) in the central dinucleotide step, compared to the only ~17-fold preference of the wild-type enzyme. To understand the basis of the increased specificity, we determined the crystal structure of NlaIV. Despite the presence of DNA in the crystallization mix, the enzyme crystallized in the free form. We therefore constructed a computational model of the NlaIV-DNA complex. According to the model, the mutagenesis of the regions that were in the proximity of DNA did not lead to the desired specificity change, which was instead conveyed in an indirect manner by substitutions in the more distant regions., (Copyright © 2019 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

Understanding chromatin dynamics is essential to define the contribution of chromatin to heritable gene silencing and the long-term maintenance of gene expression. Here we present a detailed protocol for time-ChIP, a novel method to measure histone turnover at high resolution across long timescales. This method is based on the SNAP-tag, a self-labeling enzyme that can be pulse labeled with small molecules in cells. Upon pulse biotinylation of a cohort of SNAP-tagged histones we can determine their abundance and fate across a chase period using a biotin-specific chromatin pulldown followed by DNA sequencing or quantitative PCR. This method is unique in its ability to trace the long-term fate of a chromatin bound histone pool, genome wide. In addition to a step by step protocol, we outline advantages and limitations of the method in relation to other existing techniques. time-ChIP can define regions of high and low histone turnover and identify the location of pools of long lived histones.

Pathogen-specific acquired immunity in bacteria is mediated by the CRISPR (clustered regularly interspaced short palindromic repeats)-Cas systems. Thermus thermophilus strain HB8 contains CRISPR systems of several major subtypes (type I, IIIA and IIIB), and has become a widely studied model for CRISPR biology. We have selected two highly expressed CRISPR spacers, crRNA 2.1 and crRNA 2.2, and have enriched endogenous T. thermophilus proteins that co-purify with these crRNAs. Mass spectroscopy indicates that the chromatography protocol enriches predominantly Csm complex subunits, but also Cmr subunits. After several chromatographic steps, size exclusion chromatography indicated a molecular mass of the crRNA associated complex of 265±69 kDa. In agreement with earlier work, crRNAs of different lengths (containing the selected spacers) were observed. Most of these were completely lost when several T. thermophilus csm genes were ablated.