RT Journal Article
SR Electronic
T1 SIR proteins create compact heterochromatin fibers
JF bioRxiv
FD Cold Spring Harbor Laboratory
SP 346296
DO 10.1101/346296
A1 Sarah G. Swygert
A1 Subhadip Senapati
A1 Mehmet F. Bolukbasi
A1 Scot A. Wolfe
A1 Stuart Lindsay
A1 Craig L. Peterson
YR 2018
UL http://biorxiv.org/content/early/2018/06/13/346296.abstract
AB Heterochromatin is a silenced chromatin region essential for maintaining genomic stability and driving developmental processes. The complicated structure and dynamics of heterochromatin have rendered it difficult to characterize. In budding yeast, heterochromatin assembly requires the SIR proteins -- Sir3, believed to be the primary structural component of SIR heterochromatin, and the Sir2/4 complex, responsible for the targeted recruitment of SIR proteins and the deacetylation of lysine 16 of histone H4. Previously, we found that Sir3 binds but does not compact nucleosomal arrays. Here we reconstitute chromatin fibers with the complete complement of SIR proteins and use sedimentation velocity, molecular modeling, and atomic force microscopy to characterize the stoichiometry and conformation of SIR chromatin fibers. In contrast to previous studies, our results demonstrate that SIR arrays are highly compact. Strikingly, the condensed structure of SIR heterochromatin fibers requires both the integrity of H4K16 and an interaction between Sir3 and Sir4. We propose a model in which two molecules of Sir3 bridge and stabilize two adjacent nucleosomes, while a single Sir2/4 heterodimer binds the intervening linker DNA, driving fiber compaction.