The 3' cleavage and polyadenylation of mRNAs has been studied in detail in animals and yeast, but not in plants. Aimed at elucidating the regulation of mRNA 3' end formation in plants, three Arabidopsis cDNAs encoding homologues of the animal proteins CstF-64, CstF-77 and CstF-50 that form the cleavage stimulating factor of the polyadenylation machinery have been cloned. It is shown experimentally that the N-terminal domain of the Arabidopsis CstF-64 homologue binds the mRNA 3' non-coding region in an analogous manner to the animal protein. It is also shown that the Arabidopsis CstF-64 and CstF-77 homologues strongly interact with each other in a similar way to their animal counterparts. These results imply that these Arabidopsis homologues belong to the polyadenylation machinery of nuclear mRNAs.