Cryo-EM of nucleosome core particle interactions in trans.

Nucleosomes, the basic unit of chromatin, are repetitively spaced along DNA and regulate genome expression and maintenance. The long linear chromatin molecule is extensively condensed to fit DNA inside the nucleus. How distant nucleosomes interact to build tertiary chromatin structure remains elusive. In this study, we used cryo-EM to structurally characterize different states of long range nucleosome core particle (NCP) interactions. Our structures show that NCP pairs can adopt multiple conformations, but, commonly, two NCPs are oriented with the histone octamers facing each other. In this conformation, the dyad of both nucleosome core particles is facing the same direction, however, the NCPs are laterally shifted and tilted. The histone octamer surface and histone tails in trans NCP pairs remain accessible to regulatory proteins. The overall conformational flexibility of the NCP pair suggests that chromatin tertiary structure is dynamic and allows access of various chromatin modifying machineries to nucleosomes.