DNA replication in eukaryotes initiates at many origins distributed across each chromosome. Origins are bound by the origin recognition complex (ORC), which, with Cdc6 and Cdt1, recruits and loads the Mcm2-7 (MCM) helicase as an inactive double hexamer during G1 phase. The replisome assembles at the activated helicase in S phase. Although the outline of replisome assembly is understood, little is known about the dynamics of individual proteins on DNA and how these contribute to proper complex formation. Here we show, using single-molecule optical trapping and confocal microscopy, that yeast ORC is a mobile protein that diffuses rapidly along DNA. Origin recognition halts this search process. Recruitment of MCM molecules in an ORC- and Cdc6-dependent fashion results in slow-moving ORC-MCM intermediates and MCMs that rapidly scan the DNA. Following ATP hydrolysis, salt-stable loading of MCM single and double hexamers was seen, both of which exhibit salt-dependent mobility. Our results demonstrate that effective helicase loading relies on an interplay between protein diffusion and origin recognition, and suggest that MCM is stably loaded onto DNA in multiple forms.

Accurate image alignment is critical in multicolor single-molecule fluorescence microscopy. Global alignment using affine transformations leaves residual errors due to the nonlinearity of the distortions, which decreases the effective field of view. Subsequent local refinement demands either large amounts of reference data and processing time or specialized imaging techniques like active stabilization. Here, we present a global alignment method, S/T polynomial decomposition, that uses sums of Zernike polynomial gradients to decompose the distortion between two images, correcting both linear and nonlinear distortions simultaneously. With minimal reference data, we gain diagnostic information about the distortion and achieve a colocalization accuracy comparable to local registration methods across the entire field of view.