Rate Limiting Enzymes in Nucleotide Metabolism Synchronize Nucleotide Biosynthesis and Chromatin Formation [ATAC-Seq]

Fundamental cellular processes including replication, transcription, and repair rely on both an adequate nucleotide supply and sufficient availability of new histones. Chromatin disassembly and reassembly is crucial to maintain genome stability, and 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 to complete the formation of chromatin, our knowledge about the coordination of these processes is limited. Phosphoribosyl pyrophosphate synthetases (PRPSs) catalyze the 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 the histone chaperone network involving HSP70/90, NASP, HAT1/RBBP7 and importin-4 to regulate chromatin assembly. We show PRPS enzymes are essential not only for nucleotide biogenesis, but together with PRPSAP also play a key role in the heterodimerization of H3-H4 and posttranslational modification of nascent histones, early steps in the process of histone maturation. Depletion of PRPS proteins leads to limited histone availability and therefore to impaired chromatin assembly. Our discovery bridges nucleotide metabolism and chromatin regulation and provides first evidence on how nucleotide biogenesis and chromatin dynamics work in synchrony.