Shaping the landscape: mechanistic consequences of ubiquitin modification of chromatin

The organization of eukaryotic chromosomes into transcriptionally active euchromatin and repressed heterochromatin requires mechanisms-that establish, maintain and distinguish these canonical-chromatin domains. Post-translational modifications are funda-mental in these processes. Monoubiquitylation of histones was discovered more than three decades ago, but its precise function has been enigmatic until recently. It is now appreciated that the spectrum of chromatin ubiquitylation is not restricted to mono-ubiquitylation of histones, but includes degradatory ubiquitylation of histones, histone-modifying enzymes and non-histone chromatin factors. These occur in a spatially and temporally controlled manner. In this review, we summarize our understanding of these mechanisms with a particular emphasis on how ubiquitylation shapes the physical landscape of chromatin.