The Wing Helix Domain Of Orc4 Is The Primary Determinant Of Dna Binding Specificity Of The Origin Recognition Complex

The process of DNA replication is regulated to ensure that the entire genome is replicated only once during every cell division cycle. Eukaryotic DNA replication begins with the binding of the Origin Replication Complex (ORC) to multiple replication origins or Autonomously Replicating Sequences (ARSs) on every chromosome. The ORC machinery is conserved from fungal to mammalian systems, however the ARSs to which the ORC binds have diverged significantly. In the budding yeast S. cerevisiae the ORC binds a well-defined 17bp ACS, conversely in the fission yeast S. pombe ORC binds to AT rich sequences in a stochastic manner. Previously the replication origins of the yeast K .lactis have been identified as a 50bp sequence necessary and largely sufficient for replication. Through testing of plasmids constructed to contain either S. cerevisiae or K. lactis ARSs, it was found that each species is largely unable to replicate ARSs from the other species, indicating that the replication machinery has significantly diverged to specifically recognize its own origin sequence. In this thesis, I am examining the role subunits of the ORC complex play in determining the binding specificity of the ORC complex. The ORC proteins contain a DNA binding Winged Helix Domain in their C-termini. I have constructed S. cerevisiae strains containing chimeric ORC proteins which interact with the S. cerevisiae machinery while containing the K. lactis WHD. The chimeric ORC4 and ORC5 proteins fail to substitute for their respective endogenous proteins but the ORC4 chimera results in a dominant loss of silencing at the HMR locus, which is mediated by ORC. ChIP-Seq analysis showed that the chimeric strain binds to a K. lactis ACS at this locus and at several other distinct sites in the S. cerevisiae genome including the centromeres. Additionally the chimeric ORC4 demonstrates the ability to replicate plasmids containing a K. lactis ARS, unlike the wild type S. cerevisiae strain and S. cerevisiae containing a chimeric ORC5. This study suggests that the DNA binding specificity for the S. cerevisiae ORC, K. lactis ORC and most likely ORCs in other fungi is primarily determined by the WHD of Orc4