Differential roles of positive and negative supercoiling in organizing theE. coligenome

ABSTRACT This study aims to explore whether and how positive and negative supercoiling contribute to the three-dimensional (3D) organization of the bacterial genome. We used recently published Escherichia coli GapR ChIP-seq and TopoI ChIP-seq (also called EcTopoI-seq) data, which marks positive and negative supercoiling sites, respectively, to study how positive and negative supercoiling correlates with the corresponding contact frequencies obtained from chromosome conformation capture sequencing (Hi-C and 5C). We found that supercoiled chromosomal loci have overall higher Hi-C contact frequencies than sites that are not supercoiled, with positive supercoiling surprisingly corresponding to higher spatial contacts than negative supercoiling. Additionally, Hi-C contact frequencies alone could identify positive, but not negative, supercoiling with high accuracy. The majority of positive and negative supercoils coincide with highly active transcription units, with a minor group likely associated with replication and other genomic processes. Our results suggest that both positive and negative supercoiling enhance chromosome interactions, but positive supercoils contribute more than negative supercoils to bring distant chromosomal loci closer in space. Based on these results, we propose new physical models of how the E. coli chromosome is organized differentially by positive and negative supercoils.