The ease of DNA unwinding as a determinant of initiation at yeast replication origins.

We have localized the DNA sequence that facilitates unwinding of a yeast replication origin, the H4 ARS. The readily unwound sequence lies adjacent to the previously characterized consensus core sequence of the ARS. Unwinding is detected through the formation of a single-strand-specific nuclease hypersensitive site in H4 ARS mutant derivatives present on supercoiled plasmids. Linker-scanning and linker-deletion derivatives exhibit wild-type nuclease hypersensitivity and ARS function, while large external deletions reduce or eliminate nuclease detectable unwinding and origin function. ARS unwinding and origin function can be rescued in the deletion mutants by inserting a biologically unrelated sequence with DNA unwinding properties similar to a functional ARS. The data clarify the nature of DNA sequence requirements in the ARS by suggesting that small substitutions, insertions, and deletions are tolerated in the region flanking the consensus core sequence because they do not significantly alter the unwinding properties of the region.