One of the multiple factors required for polyadenylation and 3'-end cleavage of pre-mRNAs (By similarity). This subunit is directly involved in the binding to pre-mRNAs, especially on the 3' non-coding region (PubMed:12379796). Required for the targeted 3' processing of antisense transcripts that triggers transcriptional silencing of the corresponding sense gene (PubMed:17008405, PubMed:19965720). Belongs to the CSTF complex (By similarity). Binds to mRNA. Forms a complex with cleavage and polyadenylation specificity factor (CPSF) subunits CSTF50, CSTF77, FIPS3 and FIPS5 (PubMed:12379796, PubMed:18479511, PubMed:18221017). Impaired antisense-RNA-mediated gene silencing (e.g. suppression of overexpression of FCA-mediated FLC repression). Reduced fertility, early flowering, reduced organ size and pale leaves.