Biosynthesis of histone messenger RNA employs a specific 3' end endonuclease

During S-phase of the cell cycle production of the core histone proteins is precisely balanced with DNA replication. Metazoan mRNAs encoding replication dependent (RD) histones lack polyA tail normally formed by 3' end cleavage and coupled polyadenylation of the pre-mRNA. Instead, they undergoes to endonucleolytic cleavage on the 3' side of an RNA hairpin (stem loop) producing mRNA with a 3´-stem loop (SL), which is exported from the nucleus for use in translation. The same endonuclease that is involved in normal protein-coding pre-mRNA cleavage, i.e. cleavage and poyladenylation specificity factor 73 (CPSF73), is proposed to catalyse RD pre-histone mRNA cleavage. Additional factors specific to RD pre-histone mRNA processing, including stem loop binding protein (SLBP) and the U7 small nuclear ribonucleoprotein (U7snRNP) that binds to a histone downstream element (HDE) are thought to be involved in CPSF73 targeting to RD pre-histone mRNA. We report that a different histone specific endonuclease (HSE), which like CPSF73 is a metallo ß lactamase (MBL) fold protein, is specific for RD pre-histone mRNA cleavage10,11. Crystallographic and biochemical studies reveal HSE has a di-zinc ion containing active site related to that of CPSF73, but which has distinct overall fold. Notably HSE depletion from cells leads to the production of unprocessed RD pre-histone mRNA due to inefficient 3' end processing. The consequent depletion of core histone proteins correlates with a cell cycle defect due to a delay in entering/progressing through S-phase. HSE thus may represent a new type of S-phase specific cancer target. Overall design: Examination of chromatin mRNA profiles in HeLa cells after depletion of HSE or CPSF73 by siRNA treatment.

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Pettinati I, Grzechnik P, Ribeiro de Almeida C, Brem J, McDonough MA, Dhir S, Proudfoot NJ, Schofield CJ