The half-a-tetratricopeptide repeat (HAT) motif is a helical repeat motif found in proteins that influence various aspects of RNA metabolism, including rRNA biogenesis, RNA splicing, and polyadenylation. This functional association with RNA suggested that HAT repeat tracts might bind RNA. However, RNA binding activity has not been reported for any HAT repeat tract, and recent literature has emphasized a protein binding role. In this study, we show that a chloroplast-localized HAT protein, HCF107, is a sequence-specific RNA binding protein. HCF107 consists of 11 tandem HAT repeats and short flanking regions that are also predicted to form helical hairpins. The minimal HCF107 binding site spans ∼11 nt, consistent with the possibility that HAT repeats bind RNA through a modular one repeat-1 nt mechanism. Binding of HCF107 to its native RNA ligand in the psbH 5′ UTR remodels local RNA structure and protects the adjacent RNA from exonucleases in vitro. These activities can account for the RNA stabilizing, RNA processing, and translational activation functions attributed to HCF107 based on genetic data. We suggest that analogous activities contribute to the functions of HAT domains found in ribonucleoprotein complexes in the nuclear-cytosolic compartment.helical repeat protein | plastid | pentatricopeptide repeat T he half-a-tetratricopeptide (HAT) motif is a helical repeat motif that has been functionally linked to RNA because of its presence exclusively in complexes that influence RNA metabolism (1). Examples of HAT domain proteins include Utp6, which is involved in nuclear pre-rRNA processing, Prp6, which is involved in nuclear pre-mRNA splicing, and CstF-77, which is involved in pre-mRNA cleavage and polyadenylation. The HAT motif is related to the tetratricopeptide repeat (TPR), a degenerate 34-aa motif that forms a pair of antiparallel α-helices (reviewed in ref.2). TPR motifs are typically found in tandem arrays, which stack to form a broad surface that binds protein ligands. Crystal structures of CstF-77 confirmed that HAT repeat tracts adopt a TPR-like structure (3, 4). The possibility that HAT repeat tracts might bind RNA has been suggested (1, 5-7), but the notion that HAT repeat tracts serve as a scaffold for binding other proteins dominates recent literature (3,(8)(9)(10)). This view is reflected by the annotation of the HAT motif at InterPro, which states only that "the repeats may be involved in protein-protein interactions" (http://www.ebi. ac.uk/interpro/IEntry?ac=IPR003107).Although TPR, HEAT, and several other helical repeat motifs form protein binding surfaces, similar structures have been shown to bind nucleic acids. Puf and pentatricopeptide repeat (PPR) motifs have been shown to bind RNA (reviewed in refs. 11 and 12), and mTERF, ALK, and TAL repeat motifs have been shown to bind DNA (reviewed in refs. 13 and 14). Thus, it seemed quite plausible that the presence of HAT domains exclusively in ribonucleoprotein complexes reflects the fact that HAT repeat tracts interact directly with RNA. In this ...