Earlier studies on the H4 autonomously replicating sequence (ARS) identified a DNA unwinding element (DUE), a required sequence that is hypersensitive to single-strand-specific nucleases and serves to facilitate origin unwinding. Here we demonstrate that a DUE can be identified in the C2G1 ARS, a chromosomal replication origin, by using a computer program that calculates DNA helical stability from the base sequence. The helical stability minima correctly predict the location and hierarchy of the nuclease-hypersensitive sites in a C2G1 ARS plasmid. Nucleotide-level mapping shows that the nuclease-hypersensitive site at the ARS spans a 100-base-pair sequence in the required 3'-flanking region. Mutations that stabilize the DNA helix in the broad 3'-flanking region reduce or abolish ARS-mediated plasmid replication, indicating that helical instability is required for origin function. The level of helical instability is quantitatively related to the replication efficiency of the ARS mutants. Multiple copies of either a consensus-related sequence present in the C2G1 ARS or the consensus sequence itself in synthetic ARS elements contribute to DNA helical instability. Our findings indicate that a DUE is a conserved component of the C2G1 ARS and is a major determinant of replication origin activity.