Many eukaryotic cells use RNA-directed silencing mechanisms to protect against viruses and transposons and to suppress endogenous gene expression at the posttranscriptional level. RNA silencing also is implicated in epigenetic mechanisms affecting chromosome structure and transcriptional gene silencing. Here, we describe enhanced silencing phenotype (esp) mutants inArabidopsis thalianathat reveal how proteins associated with RNA processing and 3′ end formation can influence RNA silencing. These proteins were a putative DEAH RNA helicase homologue of the yeast PRP2 RNA splicing cofactor and homologues of mRNA 3′ end formation proteins CstF64, symplekin/PTA1, and CPSF100. The last two proteins physically associated with the flowering time regulator FY in the 3′ end formation complex AtCPSF. The phenotypes of the 3′ end formationespmutants include impaired termination of the transgene transcripts, early flowering, and enhanced silencing of theFCA-β mRNA. Based on these findings, we propose that the ESP-containing 3′ end formation complexes prevent transgene and endogenous mRNAs from entering RNA-silencing pathways. According to this proposal, in the absence of these ESP proteins, these RNAs have aberrant 3′ termini. The aberrant RNAs would enter the RNA silencing pathways because they are converted into dsRNA by RNA-dependent RNA polymerases.