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Production of large amounts of histone proteins during S phase is critical for proper chromatin formation and genome integrity. This process is achieved in part by the presence of multiple copies of replication dependent (RD) histone genes that occur in one or more clusters in metazoan genomes. In addition, RD histone gene clusters are associated with a specialized nuclear body, the histone locus body (HLB), which facilitates efficient transcription and 3′ end-processing of RD histone mRNA. How all five RD histone genes within these clusters are coordinately regulated such that neither too few nor too many histones are produced, a process referred to as histone homeostasis, is not fully understood. Here, we explored the mechanisms of coordinate regulation between multiple RD histone loci in Drosophila melanogaster and Drosophila virilis. We provide evidence for functional competition between endogenous and ectopic transgenic histone arrays located at different chromosomal locations in D. melanogaster that helps maintain proper histone mRNA levels. Consistent with this model, in both species we found that individual histone gene arrays can independently assemble an HLB that results in active histone transcription. Our findings suggest a role for HLB assembly in coordinating RD histone gene expression to maintain histone homeostasis. 

Splicing is highly regulated and is modulated by numerous factors. Quantitative predictions for how a mutation will affect precursor mRNA (pre-mRNA) structure and downstream function are particularly challenging. Here, we use a novel chemical probing strategy to visualize endogenous precursor and mature MAPT mRNA structures in cells. We used these data to estimate Boltzmann suboptimal structural ensembles, which were then analyzed to predict consequences of mutations on pre-mRNA structure. Further analysis of recent cryo-EM structures of the spliceosome at different stages of the splicing cycle revealed that the footprint of the B act complex with pre-mRNA best predicted alternative splicing outcomes for exon 10 inclusion of the alternatively spliced MAPT gene, achieving 74% accuracy. We further developed a β-regression weighting framework that incorporates splice site strength, RNA structure, and exonic/intronic splicing regulatory elements capable of predicting, with 90% accuracy, the effects of 47 known and 6 newly discovered mutations on inclusion of exon 10 of MAPT . This combined experimental and computational framework represents a path forward for accurate prediction of splicing-related disease-causing variants.