Data from: The Origin Recognition Complex requires chromatin tethering by a hypervariable intrinsically disordered region that is functionally conserved from sponge to man

The first step towards eukaryotic genome duplication is loading of the
replicative helicase, the Mcm2-7 complex, onto chromatin. This so-called
“licensing” step is executed by the Pre-Replication Complex (Pre-RC) whose
assembly on chromatin is nucleated by the DNA-binding activity of the
Origin Recognition Complex (ORC). It is thought that metazoan ORC, like
the yeast complex, is recruited directly to chromatin by its ATP-dependent
DNA binding and encirclement activity. However, we have previously shown
that this DNA binding mode is dispensable for chromatin recruitment of fly
ORC, raising the question of how metazoan ORC binds chromosomes. We show
here that the intrinsically disordered region (IDR) of fly Orc1 is both
necessary and sufficient for the recruitment of ORC to chromosomes in vivo
and demonstrate that this activity is regulated by IDR phosphorylation. In
vitro studies show that the IDR alone binds DNA and this bestows the ORC
holocomplex with a high-affinityATP-independentDNA binding mode.
Interestingly, we find that Orc1 IDRs have diverged so markedly across
metazoans that they are unrecognizable as orthologs and yet we find that
these compositionally homologous sequences retain DNA and chromatin
binding activity down to basal metazoans. Altogether, these data suggest
that chromatin is recalcitrant to ORC’s ATP-dependent DNA binding activity
and we propose that this necessitates IDR-dependent chromatin tethering
which poises ORC to opportunistically encircle nucleosome-free regions as
they become available. This work reveals a novel step in metazoan
replication licensing and expands our understanding of disordered protein
homology and evolution by stretching the relationship between primary
structure and function.