Component of the cleavage and polyadenylation specificity factor (CPSF) complex that play a key role in pre-mRNA 3'-end formation. May interact with poly(A) polymerase and other factors to bring about cleavage and poly(A) addition (By similarity). Mediates poly(A) site selection (PubMed:23136375). Binds RNA in a calcium-dependent manner (PubMed:16500995, PubMed:17576667, PubMed:20214900). Exhibits endonuclease activity with an ability to nick and degrade linear as well as circular single-stranded RNA that leaves RNA 3' ends with hydroxyl groups, thus mediating processing of the pre-mRNA as a prelude to the polyadenylation (PubMed:17576667). Involved in the post-transcriptional control, probably via poly(A) addition, of the responses of plants to stress, especially genes mediating tolerance to oxidative stress (PubMed:18545667). Plays a role in the regulation of salicylic acid (SA) production via the control of messenger RNA 3' end processing, thus being a key component of programmed cell death and plant immune responses required for resistance to virulent Pseudomonas syringae pv tomato DC3000 (Pst) (PubMed:24706550). Endonuclease activity is repressed by the N-terminal domain of FIPS5 (PubMed:17576667). Nuclease activity is inhibited by zinc (>100 uM), cadmium in a progressive manner (50 percent activity at 1 mM Cd(2+)), and high salt levels (e.g. KCl or NaCl >600 mM). Stimulated by ATP in the presence of Zn(2+), even at inhibitory zinc concentrations. Elevated temperatures prevent RNA-binding at 55 degrees Celsius, but endonuclease activity at 70 degrees Celsius. The sulfhydryl reagent dithiothreitol (DTT) inhibits both RNA-binding and nuclease activities (PubMed:18331819). Component of the cleavage and polyadenylation specificity factor (CPSF) complex (Probable). Can form homodimers (PubMed:18479511). Binds to calmodulin (PubMed:16500995, PubMed:16897494). Forms a complex with cleavage and polyadenylation specificity factor (CPSF) subunits CPSF73-I, CPSF73-II, CPSF100, CPSF160, CFIS2, FIPS3, FIPS5, PAPS2, PAPS3, CLPS3, PCFS1, PCFS4, CSTF50 and CSTF77 (PubMed:16282318, PubMed:18479511, PubMed:20214900, PubMed:17576667, PubMed:19573236). Localized in the cytoplasm when not in complex or when associated with CPSF100, but move to the nucleus when associated with the cleavage and polyadenylation specificity factor (CPSF) subunits CPSF160 or CPSF73s. Expressed in seedlings, roots, leaves, siliques, stems and flowers. Isoform 2 is up-regulated by exposure to the oxidative agent methyl viologen (MV). In oxt6, small plants, especially at temperatures above 22 degrees Celsius. Enhanced tolerance to oxidative stress associated with elevated constitutive expression of genes that encode proteins containing thioredoxin- and glutaredoxin- related domains (PubMed:18545667). Altered poly(A) site choice (PubMed:18545667, PubMed:23136375). Suppresses cell death in lesion-mimic mutants (e.g. mips1, lsd1, mkk4, cpr5, and cat2). Enhanced sensitivity to virulent Pseudomonas syringae pv tomato DC3000 (Pst) (PubMed:24706550). Belongs to the CPSF4/YTH1 family.