Genomic looping: a key principle of chromatin organization.

The effective volume occupied by the genomes of all forms of life far exceeds that of the cells in which they are contained. Therefore, all organisms have developed mechanisms for compactly folding and functionally organizing their genetic material. Through recent advances in fluorescent microscopy and 3C-based technologies, we finally have a first glimpse into the complex mechanisms governing the 3-D folding of genomes. A key feature of genome organization in all domains of life is the formation of DNA loops. Here, we describe the main players in DNA organization with a focus on DNA-bridging proteins. Specifically, we discuss the properties of the bacterial DNA-bridging protein H-NS. Via two different modes of binding to DNA, this protein is a key driver of bacterial genome organization and provides a link between 3-D organization and transcription regulation. Importantly, H-NS function is modulated in response to environmental cues, which are translated into adapted gene expression patterns. We delve into the mechanisms underlying DNA looping and explore the complex and subtle modulation of these diverse, yet difficult-to-study, structures. DNA looping is universal and a conserved mechanism of genome organization throughout all domains of life.