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Abstract Ribonuclease (RNase) MRP is a conserved RNA-based enzyme best known for its essential role in the maturation of ribosomal RNA (rRNA) in eukaryotes. However, the composition and RNA substrate specificity of this multisubunit ribonucleoprotein complex in higher eukaryotes remain a mystery. Here, we identify NEPRO and C18ORF21 (which we renamed RMP64 and RMP24, respectively) as constitutive subunits of metazoan RNase MRP. These proteins are unique to RNase MRP and absent from the closely related RNase P, which processes transfer RNA (tRNA) precursors and tRNA-like substrates. We find that RMP64 and RMP24 are integral subunits of RNase MRP, stabilize its catalytic RNA, and are required for rRNA maturation and cell proliferation. Leveraging these discoveries, we identify a broad suite of in vivo RNA-binding targets of each enzyme, including potential cleavage sites at nucleotide resolution. Our findings identify the first metazoan RNase MRP-specific protein subunits and define the RNA-targeting repertoire of this essential enzyme in mammalian cells. 

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.