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Posttranslational modifications of histones such as H3K4 trimethylation and H2B monoubiquitination show a positive correlation with transcription activation. Until now, all H2B monoubiquitination was thought to arise from the action of the heterodimeric RNF20/40 E3 ubiquitin ligase. Li et al. (p. 1673-1688) report a new histone-modifying activity associated with the mammalian chromatin remodeling complex SWI/SNF-A. The study finds that the BAF250 subunit associates with elongin C, cullin 2, and Roc1 to assemble a complex that monoubiquitinates histone H2B at K120. This work has implications for the concerted action of chromatin remodeling and modifying activities in regulating target gene expression.
Estrogen induces proliferation of the uterine epithelial lining in preparation for pregnancy. Female mice lacking the transcription factor C/EBPβ display a profound defect in this hormone-dependent process and are infertile. A study by Ramathal et al. (p. 1607-1619) revealed that C/EBPβ controls the expression of key cell cycle factors regulating DNA replication in uterine epithelial cells. In replication-defective C/EBPβ-null epithelial cells, cell cycle arrest activates classical DNA damage response pathways, triggering p53-mediated mechanisms that promote cell death. C/EBPβ, therefore, is an essential mediator of estrogen-induced growth and survival of the uterine epithelial cells during a critical phase of female reproduction.