We report here the identification and characterization of two novel proteis from Saccharomyces cerevisiae that bind to the T-rich strand of the core consensus autonomously replicating sequence (ARS) in a highly specific manner. The two proteins, 40 and 45 kDa in size, can be distinguished by multiple criteria from each other and from the 65-kDa ssArS-T-binding protein identified recently in our laboratory (Schmidt, A. M. A., Herterich, S. U., and Krauss, G. (1991) EMBO J. 10, 981-985). The specificity of binding is inferred from gel shift and nuclease-footprinting experiments using single-stranded probes containing the core consensus ARS. With a 321-nucleotide single-stranded ARS1 fragment, specific protection of the A and B1 domain against DNase I digestion is observed. Partial amino acid sequencing and enzymatic assays identify the 45-kDa protein as adenylosuccinate synthetase, an enzyme necessary for the de novo synthesis of adenylate.