RIZ1 is a transcriptional regulator and tumor suppressor that catalyzes methylation of lysine 9 of histone H3. It contains a distinct SET domain, sometimes referred to as PR (PRDI-BF1 and RIZ1 homology) domain, that is responsible for its catalytic activity. We determined the solution structure of the PR domain from RIZ1 and characterized its interaction with S-adenoSyl-L-homocysteine (SAH) and a peptide from histone H3. Despite low sequence identity with canonical SET domains, the PR domain displays a typical SET fold including a pseudo-knot at the C-terminus. The N-flanking sequence of RIZ1 PR domain adopts a novel conformation and interacts closely with the SET fold. The C-flanking sequence contains an alpha-helix that points away from the protein face that harbors active site in other SET domains. The SET fold of RIZ1 does not have detectable affinity for SAH but it interacts with a synthetic peptide comprising residues 1-20 of histone H3. (c) 2007 Elsevier Inc. All rights reserved.