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A subset of poly(A) factors is required for histone pre‐mRNA processing in D. melanogaster

Metazoan histone pre‐mRNAs end in a stem‐loop (SL) structure rather than a poly(A) tail. Formation of histone mRNAs requires cleavage directed by the SL, which binds SLBP, and a histone downstream element, which is recognized by U7 snRNP. In D. melanogaster, poly(A) signals located downstream of the normal histone pre‐mRNA cleavage sites are utilized when cleavage is inhibited. Knockdown of CPSF73, CPSF100, and Symplekin results in polyadenylation of histone mRNA, while depletion of CstF50, 64 and 77, CPSF 30 and 160 does not affect processing, suggesting that CPSF73, CPSF100 and Symplekin comprise the core of a common cleavage factor. Depletion of Symplekin, CPSF73, or CPSF100 results in reciprocal co‐depletion of the other two factors. Each of these factors coimmunoprecipitates the other two. Lsm11 and SLBP also coIP with CPSF73 and Symplekin, indicating that the U7 snRNP and SLBP may recruit the cleavage complex. ChIP experiments show that both Symplekin and CPSF73 associate with D. melanogaster H1 and H3 genes, while CstF50 does not. Depletion of SLBP by RNAi results in association of RNA Pol II, CPSF73, Symplekin and CstF50 with the region between the H1 and H3 genes. These results suggest that CstF is not associated with the histone cleavage factor in vivo and support a model in which there is a distinct cleavage factor for histone pre‐mRNA processing which shares only some components with the poly(A) cleavage factor.