Recognition of ubiquitinated nucleosomes.

Histone ubiquitination plays a non-degradative role in regulating transcription and the DNA damage response. A mechanistic understanding of this chromatin modification has lagged that of small histone modifications because of the technical challenges in preparing ubiquitinated nucleosomes. The recent structure of the DUB module of the SAGA coactivator complex bound to a nucleosome containing monoubiquitinated H2B has provided the first view of how specialized subunits target this enzyme to its substrate. Single particle electron microscopy of the intact SAGA coactivator suggests how the DUB module and histone acetyltransferase module engage a nucleosomal substrate. A cryo EM study of 53BP1 bound to nucleosomes containing ubiquitinated H2A and H4 methylated at K20 extends our understanding of recognition of biologically distinct combinations of chromatin marks through multivalent interactions.