For many human protein-coding genes, alternative cleavage and polyadenylation (APA) of pre-mRNA generates distinct 3' untranslated regions (3'UTRs) with differing regulatory potential. Widespread 3'UTR shortening via APA occurs in proliferative cell states, including cancer, where it can lead to oncogene overexpression. There has therefore been significant interest in identifying factors which influence poly(A) site choice in different physiological states. The multi-subunit human cleavage factor I complex (CFIm), a core component of the mammalian pre-mRNA cleavage machinery, has been identified as a potential master regulator of APA, as its depletion leads to widespread 3'UTR shortening. However, mechanistic understanding of how CFIm influences poly(A) site selection, and how its activity is regulated, is lacking. In this work, gene editing was used to generate cell lines with substantial, permanent depletion of the 25 kDa or 68 kDa subunits of CFIm (CFIm25 and CFIm68), which exhibited the expected 3'UTR shortening for representative transcripts. Reversal of this 3'UTR shortening by CFIm25 or CFIm68 re-expression provided the basis for a complementation assay, which allowed various aspects of CFIm25 and CFIm68 function to be investigated in vivo. The capacity of CFIm25 to recognise UGUA RNA sequences was shown to make an important contribution to poly(A) site selection transcriptome-wide, and a novel function for the C-terminal arginine/serine-rich (RS domain) of CFIm68 in poly(A) site selection was identified. The potential contribution of CFIm post-translational modification (PTM) to APA regulation was also explored. Novel acetylation sites on CFIm25 and CFIm68 were identified, as well as extensive serine phosphorylation in the CFIm68 RS domain. Complementation analysis revealed that phosphomimetic mutations in this RS domain inhibited distal poly(A) site selection, suggesting a potential role for CFIm68 phosphorylation in APA regulation. Taken together, the findings presented here provide insights into several important determinants of CFIm function, and the complementation assay developed provides a useful tool for future investigations.

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