Spatially correlated fluctuations govern relative chromatin motion
Essential nuclear processes require pairs of chromosomal loci to find each other in three-dimensional space. Polymer models of chromosome dynamics typically assume that the stochastic forces driving such locus motion are spatially uncorrelated, implying that relative diffusion follows directly from single-locus dynamics. Here we show that this assumption fails in living cells. Using live-cell imaging in fly embryos and mouse embryonic stem cells, we find that pairwise locus distances diffuse markedly slower than predicted for independent fluctuations. Combining stochastic trajectory analysis with polymer simulations, we demonstrate that this slowdown arises from non-equilibrium spatially correlated fluctuations (SCFs) in the nucleoplasm, which cause nearby loci to move coherently. We establish three experimentally testable signatures of SCFs: fluctuation amplitudes plateau at large distances, are independent of genomic separation, and show an anomalous temporal scaling. All three predictions are confirmed experimentally, including for loci on separate chromosomes. ATP depletion and disruption of cohesin-mediated loop extrusion reveal that both active processes and crosslinking contribute to correlation magnitudes. Because SCFs slow relative motion preferentially at short distances, they reduce encounter frequencies while prolonging encounter durations, generating a trade-off with direct implications for gene regulation. Our results identify spatially correlated fluctuations as a fundamental determinant of relative motion in confined active polymers.