Heterochromatin assembly by interrupted Sir3 bridges across neighboring nucleosomes.
Heterochromatin is a conserved feature of eukaryotic chromosomes with central roles in regulation of gene expression and maintenance of genome stability. Heterochromatin formation involves spreading of chromatin-modifying factors away from initiation points over large DNA domains by poorly understood mechanisms. InSaccharomyces cerevisiae, heterochromatin formation requires the SIR complex, which contains subunits with histone-modifying, histone-binding, and self-association activities. Here, we analyze binding of the Sir proteins to reconstituted mono-, di-, tri-, and tetra-nucleosomal chromatin templates and show that key Sir-Sir interactions bridge only sites on different nucleosomes but not sites on the same nucleosome, and are therefore 'interrupted' with respect to sites on the same nucleosome. We observe maximal binding affinity and cooperativity to unmodified di-nucleosomes and propose that nucleosome pairs bearing unmodified histone H4-lysine16 and H3-lysine79 form the fundamental units of Sir chromatin binding and that cooperative binding requiring two appropriately modified nucleosomes mediates selective Sir recruitment and spreading.
Department of Cell Biology, Howard Hughes Medical Institute, Harvard Medical School, Boston, United States.
Reza
0000-0003-3064-9743
R
Behrouzi
Department of Cell Biology, Howard Hughes Medical Institute, Harvard Medical School, Boston, United States.
Chenning
C
Lu
Department of Cell Biology, Howard Hughes Medical Institute, Harvard Medical School, Boston, United States.
Mark A
MA
Currie
Department of Cell Biology, Howard Hughes Medical Institute, Harvard Medical School, Boston, United States.
Gloria
G
Jih
Department of Cell Biology, Howard Hughes Medical Institute, Harvard Medical School, Boston, United States.
Nahid
N
Iglesias
Department of Cell Biology, Howard Hughes Medical Institute, Harvard Medical School, Boston, United States.
Danesh
0000-0003-0321-6221
D
Moazed
D006570:Heterochromatin / Q000378:metabolism*; D009707:Nucleosomes / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000235:genetics* / Q000378:metabolism*; D038281:Silent Information Regulator Proteins, Saccharomyces cerevisiae / Q000378:metabolism*
D006570:Heterochromatin; D009707:Nucleosomes; C084122:SIR3 protein, S cerevisiae; D038281:Silent Information Regulator Proteins, Saccharomyces cerevisiae
S. cerevisiae; Sir3; Sir4; biochemistry; chromatin spreading; chromosomes; genes; heterochromatin; nucleosome