Human initiation protein Orc4 prefers triple stranded DNA

In higher eukaryotes mechanism of DNA replication origin recognition and binding by origin recognition complex (ORC) is still unknown. Origin transfer studies have shown that origin sites are genetically determined, containing functionally interchangeable modules. One of such modules from the human lamin B2 origin of replication has the ability to adopt unorthodox structure partly composed of intramolecular triplex. Sequences involved in triplex formation coincide with ORC binding sites both in vitro and in vivo. To explore potential significance of unorthodox DNA structures in origin recognition by ORC, we tested DNA binding properties of human ORC subunit 4 (HsOrc4) which has independent DNA binding activity in vitro and similar binding characteristics as ORC holocomplex. Our results demonstrated that DNA binding activity of HsOrc4 depends on length and structure of DNA with triplex being the protein's preferred binding target. Such feature could play part in origin selection through directing ORC to DNA sequence prone to adopt unorthodox structure.