Replication of chromosomal DNA in eukaryotes has two major stages.  Starting in the G1 phase of the cell cycle, double hexamers consisting of two copies of the Mcm2-7 replicative helicase are assembled at replication origins.   Later, in S phase, the two helicases are incorporated into two oppositely oriented CMG (Cdc45-Mcm2-7-GINS) complexes that each then form the core of a replisome.  Control of this “activation” step, which is triggered by the protein kinases DDK and S-CDK, is essential to ensure that each part of the genome is replicated once and only once in each cell cycle.

In this paper, Lorraine De Jesús-Kim together with collaborators from Steve Bell’s and Jeff Gelles’ labs used multi-wavelength single-molecule fluorescence colocation  (“CoSMoS”) methods to study in vitro the molecular mechanism of the activation process.  The journal’s acceptance summary notes that “The manuscript provides new and convincing evidence that a heretofore unknown intermediate state [called “CtG”] for replication start contains multiple copies of the GINS and Cdc45 proteins prior to initiation at each origin with one double hexamer of the MCM2-7 complex. The number of GINS and Cdc45 is determined by DDK phosphorylation of the MCM’s and the probability to create an active helicase (CMG) is increased with multiple numbers of the bound ancillary factors…. The single molecule studies and biochemistry are beautifully executed providing the evidence for such intermediates…. The addition of in vivo studies demonstrates that modulating the multiplicity of DDK phosphorylation (and proposed, CtG formation) has an impact on origin usage in cells.”