The yeast ARS binding factor 1 (ABF1)--where ARS is an autonomously replicating sequence--and repressor/activator protein 1 (RAP1) have been implicated in DNA replication, transcriptional activation, and transcriptional silencing. The ABF1 gene was cloned and sequenced and shown to be essential for viability. The predicted amino acid sequence contains a novel sequence motif related to the zinc finger, and the ABF1 protein requires zinc and unmodified cysteine residues for sequence-specific DNA binding. Interestingly, ABF1 is extensively related to its counterpart, RAP1, and both proteins share a region of similarity with SAN1, a suppressor of certain SIR4 mutations, suggesting that this region may be involved in mediating SIR function at the silent mating type loci.