Histone mRNA Regulation in Development

Project Summary/Abstract Metazoan replication-dependent histone mRNAs are the only eukaryotic mRNAs that lack a polyA tail ending instead in a conserved stemloop. In contrast mRNAs for histone variants, e.g. H3.3 and H2A.Z, are encoded by polyadenylated mRNAs. The genes for all five histone proteins are clustered in metazoan genomes, and factors required for histone gene expression are concentrated at the histone genes in a nuclear body, the histone locus body (HLB) which contains three factors (FLASH, Mxc (NPAT) and U7 snRNP) that only function in histone mRNA biosynthesis. Histone mRNAs are synthesized in the HLB. Our goal is to understand how the HLB is assembled and concentrates the factors required for histone mRNA biosynthesis, and activates expression only during S-phase. We have shown the core U7 snRNP is targeted to the HLB in G1, but the active U7 snRNP is only assembled in the HLB in S-phase cells, when the cleavage factor containing CPSF73, CPSF100 and symplekin is bound to U7 snRNP in the HLB. The HLB forms immediately after mitosis and is inactive until cells enter S-phase. Phosphorylation of Mxc (NPAT) by cyclin E/cdk2 activates histone gene expression. In this proposal we address how U7 snRNP is targeted to the HLB (aim 1), how U7 snRNP is activated inside the HLB (aim 2) and using the ability to remove specific factors from the nucleus during early embryogenesis to determine the role of other factors in histone mRNA biosynthesis (aim 3).