Two factors that interact specifically with the chromosomal replicator, autonomously replicating sequence 1 (ARS1), were identified in whole-cell yeast extracts. One of these proteins, ARS binding factor I (ABF-I), was purified to homogeneity as a polypeptide of 135 kDa. ABF-I binds within a region previously shown to be essential for the function of ARS1 when yeast are grown under certain nutritional conditions, and deletion analysis presented here suggests that the ARS1 domain containing the ABF-I-binding site, domain B, is a complex element composed of at least two subdomains. ABF-I also binds within an ARS at the HMR silent mating locus. This ARS functions as a cis-acting transcriptional silencer, and the ABF-I-binding site within this ARS is important for both ARS and silencer function.