The interaction between silence information regulator 1 protein (Sir1p) and origin recognition complex 1 protein (Orc1p), the largest subunit of the origin recognition complex, plays an important role in the establishment of transcriptional silencing at the cryptic mating-type gene loci in Saccharomyces cerevisiae. Sir1p binds the N-terminal region of Orc1p encompassing a Bromoadjacent homology (BAH) domain found in various chromatinassociated proteins. To understand the molecular mechanism of Sir protein recruitment, we have determined a 2.5-Å cocrystal structure of the N-terminal domain of Orc1p in complex with the Orc1p-interacting domain of Sir1p. The structure reveals that Sir1p Orc1p-interacting domain has a bilobal structure: an ␣͞ N-terminal lobe and a C-terminal lobe resembling the Tudor domain royal family fold. The N-terminal lobe of Sir1p binds in a shallow groove between a helical subdomain and the BAH domain of Orc1p. The structure provides a mechanistic understanding of Orc1p-Sir1p interaction specificity, as well as insights into protein-protein interactions involving BAH domains in general.structure ͉ transcriptional silencing E pigenetic control of gene expression involves the assembly of higher order chromatin structures. In the budding yeast Saccharomyces cerevisiae, cryptic mating-type genes, HML and HMR, are epigenetically silenced. Genetic and biochemical studies have identified cis-and trans-acting factors required for the establishment and maintenance of transcriptional silencing at the HM loci (1). The silent HM loci are flanked by specific cis-acting DNA sequences called the E and I silencers, which contain two or more binding sites for DNA-binding proteins, the origin recognition complex (Orc), Rap1p, and Abf1p (2-6). Silencing at the HM loci also requires four silent information regulator (Sir) proteins, Sir1p, Sir2p, Sir3p and Sir4p (7,8).Orc is a six-protein complex important for initiation of DNA replication and transcriptional silencing (2, 9, 10). The Nterminal domain (NTD) of Orc1p, the largest subunit of Orc, is specifically required for transcriptional silencing at the HM loci (11). The NTD of Orc1p is Ϸ220 residues in length, and it shares Ϸ50% amino acid identity with the N-terminal region of Sir3p. The NTD of Orc1p interacts with Sir1p, and this interaction is important for recruiting other SIR proteins to the HM loci as Sir1p also interacts with Sir4p (12). In HM and telomeric silencing, Sir4p forms a complex with Sir2p, and the Sir2p͞Sir4p complex is joined by . Sir2p is a NAD-dependent histone deacetylase (16-18), whereas Sir3p and Sir4p appear to have structural roles. Sir3p and Sir4p can self-associate, interact with Rap1p, and bind to the N-terminal tails of histone H3 and H4 (19, 20). This complex network of protein-protein interactions is responsible for the assembly of repressive higher-order chromatin structures at the HM loci.The NTD of Orc1p contains a bromo-adjacent homology (BAH) domain (21), which is also present in a number of other chromatin-associated p...