Anchoring of parasitic plasmids to inactive regions of eukaryotic chromosomes through nucleosome signal

Abstract

Natural plasmids are common in prokaryotes but few have been documented in eukaryotes. The natural 2µ plasmid present in budding yeastSaccharomyces cerevisiaeis one of the most well characterized. This highly stable genetic element coexists with its host for millions of years, efficiently segregating at each cell division through a mechanism that remains poorly understood. Using proximity ligation (Hi-C, Micro-C) to map the contacts between the 2µ and yeast chromosomes under dozens of different biological conditions, we found that the plasmid tether preferentially on regions with low transcriptional activity, often corresponding to long inactive genes. Common players in chromosome structure such as members of the structural maintenance of chromosome complexes (SMC) are not involved in these contacts which depend instead on a nucleosomal signal associated with a depletion of RNA Pol II. These contacts are stable throughout the cell cycle, and can be established within minutes. This strategy may involve other types of DNA molecules and species other thanS. cerevisiae, as suggested by the binding pattern of the natural plasmid along the silent regions of the chromosomes ofDictyostelium discoideum.