Formation of noncanonical DNA structures mediated by human ORC4, a protein component of the origin recognition complex

Many genomic sequences, DNA replication origins included, contain specific structural motifs prone to alternative base pairing. Structural rearrangements of DNA require specific environmental conditions and could be favored by chemical agents or proteins. To improve our understanding of alternative conformations of origins and the manner in which they form, we have investigated the effect of DNA-binding, AAA+ protein human ORC4 on single-stranded origin DNA or various oligonucleotides. Here we demonstrate that human ORC4 stimulated formation of inter- and intramolecular T.A.T triplexes and created novel structures, such as homoadenine duplexes. Adenine-based structures were held together by Hoogsteen hydrogen bonds, as demonstrated on 7-deaza-dAMP- or dAMP-containing substrates, and characterized by increased thermal stability. Adenine pairing occurred only in the presence of human ORC4, in a neutral buffer supplemented with ATP and Mg (2+) ions. The protein mutant that could not bind ATP was inactive in this reaction. Since the action of human ORC4 could be biologically important, its potential impact on DNA replication is discussed.