Mechanisms mediating cyclin E/CDK2-regulated replication-dependent histone mRNA biosynthesis.

摘要

Nuclear organization is a dynamic modulator of gene expression. Subcellular compartments called nuclear bodies concentrate factors regulating gene expression and change in size, composition, and activities in response to cellular inputs. Thus, understanding the mechanisms that assemble and organize nuclear bodies is important for appreciating how they contribute to genome function. We explored the relationship between histone locus bodies (HLBs) and replication-dependent histone mRNA biosynthesis as a model for understanding nuclear body function. HLBs are localized near the histone genes and are enriched with factors required for histone mRNA biosynthesis. Cyclin E/Cdk2 is necessary for cell cycle-dependent histone mRNA biosynthesis. However, the molecular mechanisms of this regulation are not fully known. Using the MPM-2 monoclonal antibody as a tool for exploring cell cycle-mediated control of HLB function, we show that in Drosophila cells MPM-2 detects Cyclin E/Cdk2-dependent nuclear foci that co-localize with nascent histone transcripts and are coincident with HLBs. To identify MPM-2 reactive HLB proteins, we performed a genome-wide RNAi screen and used mass spectroscopy to identify novel HLB proteins. We show that one of these factors, Mxc, is MPM-2 reactive. We propose that Cyclin E/Cdk2 regulates histone mRNA biosynthesis by modulating the activity of Mxc. Interestingly, Mxc is required for HLB assembly and efficient histone pre-mRNA processing. These results suggest that Cyclin E/Cdk2 employs multiple levels of control to ensure that histone gene expression and DNA replication are properly coordinated during cell division.