Rate Limiting Enzymes in Nucleotide Metabolism Synchronize Nucleotide Biosynthesis and Chromatin Formation.

Chromatin formation requires both an adequate nucleotide supply and sufficient availability of histones. Chromatin must be assembled following DNA-based fundamental cellular processes such as replication and transcription to preserve genome integrity. Chromatin assembly is regulated by the orderly engagement of histones with a series of histone chaperones that guide newly synthesized histones from ribosomes to DNA. Although the synthesis of nucleotides and the histone proteins are the two major biosynthetic processes that complete the formation of chromatin, how these processes are coordinated remains unknown. Phosphoribosyl pyrophosphate synthetases (PRPSs) catalyze the first and rate-limiting step in the nucleotide biosynthesis pathway. PRPS enzymes form a complex with PRPS-associated proteins (PRPSAPs). In the present study, we discover that PRPS-PRPSAP enzyme complex are part of histone chaperone network. We show that PRPS enzymes are essential not only for nucleotide biogenesis, but together with PRPSAP also play a key role in the early steps in the process of histone maturation by regulating the interaction of histone chaperones with histone H3 and H4. Importantly, this regulation is separate from PRPS nucleotide biosynthetic activity. Depletion of PRPS proteins leads to limited histone availability and impaired chromatin assembly. Our discovery bridges cellular metabolism and chromatin regulation and provides the evidence of how nucleotide biogenesis and histone deposition are coordinated.