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Bacterial DNA-recognition and DNA-modifying systems provide ways to mark, map and perturb <eukaryotic> <chromosomes>. Their value comes from separating the targeting module from the reporter or effector, so a <chromosome> becomes experimentally observable without assuming that its sequence alone specifies its behaviour.
For <fluorescent operator-repressor chromosome labelling>, an integrated array of bacterial operator sequences is bound by a fluorescent repressor fusion. A <LacI> fusion bound to a <lac operator> array makes a selected <genetic locus> visible in a living nucleus. Two differently labelled loci can report separation of <sister chromatids>, movement relative to the nuclear periphery, or distributions of distances between <chromosome> regions. <Carmen C. Robinett> and colleagues demonstrated operator-based imaging in mammalian cells and yeast. The tag reports the marked locus, not every part of the <chromosome>, and repeated operators or excessive bound repressor can perturb <DNA replication> and <chromatin> structure.<Lac-operator chromosome imaging study by Carmen C. Robinett and others>[Live GFP-repressor imaging and targeted yeast insertion]{ref}
The bacterial <CRISPR-Cas9> recognition mechanism makes targeting programmable. A fluorescent <dCas9> fusion and a <guide RNA> can visualize native repeated sequences; multiple guides can improve detection of a nonrepetitive locus. <Baohui Chen> and colleagues used this approach to follow human <genetic loci>. Catalytically inactive targeting avoids making the <DNA double-strand break> that would itself alter <chromosome> behaviour. Appropriate controls include guide specificity, signal from an unlabelled locus, and whether tagging changes growth or the measured dynamics.<CRISPR imaging of human genomic loci by Baohui Chen and others>[Live imaging of endogenous human loci]{ref}
For <DamID>, bacterial Dam methyltransferase is fused to a nuclear <protein>. Methylated GATC sites record neighbourhoods visited by the fusion. A fusion to a nuclear-envelope component can identify <lamina-associated domains>, whereas one to a chromatin-binding <protein> maps its genomic neighbourhoods. Comparing with untethered Dam helps control accessibility-dependent background. The accumulated methylation signal is a time-integrated proximity record: it need not establish direct binding to every recovered sequence, and resolution depends on GATC distribution and local spreading.<Identification of in vivo DNA targets of chromatin proteins using tethered Dam methyltransferase by Bas van Steensel and Steven Henikoff>[Principle of targeted methylation and chromatin-protein mapping]{ref}
Bacterial enzymes also make structural measurements possible. In <Hi-C>, <restriction enzymes> generate <DNA> fragments whose proximity-dependent ligation junctions can be sequenced. In <ATAC-seq>, a modified bacterial <Tn5 transposase> inserts sequencing adapters preferentially into accessible <DNA>. Neither assay directly photographs a fixed <chromosome> structure: contact frequency and accessibility are different observables. Enzyme sequence preferences, fixation, and cell-to-cell heterogeneity must be controlled. Together, live labelling, proximity recording and sequencing-based assays connect <chromosome> movement, nuclear neighbourhood and <chromatin accessibility>, rather than substituting one measurement for all three.
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