Differential control of both cell cycle-regulated and quantitative histone mRNA expression by Drosophila Mute.

Coupling histone gene expression to S phase of the cell cycle is essential for genome duplication and stability. Activation of Cyclin E/Cdk2 at the G1-S transition stimulates high-level expression of histone genes during S phase, but how histone genes are turned off at the end of S phase is not understood. Here we demonstrate that the essential Drosophila gene mute functions to repress inappropriate histone mRNA accumulation outside of S phase by counteracting Cyclin E/Cdk2-dependent phosphorylation of Mxc, which activates histone gene expression. Additionally, Mute plays contrasting roles in histone gene expression during S phase by promoting high levels of H1, H2a and H2b expression but not H3 and H4. Although Mute is present only at replication-dependent histone genes, its loss leads to 801 differentially regulated genes, primarily those involved in muscle related processes in late-stage embryos. Thus, disruptions of histone gene expression control alters the transcriptome resulting in developmental defects.