Ensemble Assembly and Activation of Fluorescently Labeled CMG onto Magnetic Bead-Bound DNA for Single Molecule Imaging

To begin, take one 75 microliter aliquot containing one milligram of DNA-bound streptavidin-coated magnetic beads and remove the supernatant with the magnetic rack. Wash the beads with 200 microliters of loading buffer. Then resuspend the beads in 75 microliters of loading buffer and mix gently by pipetting.

Next, add the origin recognition complex at a final concentration of 37.5 nanomolar to the bead-bound DNA and incubate the reaction for five minutes at 30 degrees Celsius with 800 revolutions per minute agitation. Add CDC6 at a final concentration of 50 nanomolar and incubate the reaction as demonstrated before. Then add Mcm2-7 Cdt1 at a final concentration of 100 nanomolar and incubate the reaction for 20 minutes as demonstrated previously.

After that, add DDK at a final concentration of 100 nanomolar and incubate similarly for 30 minutes. After removing the supernatant, wash the bead-bound DNA containing phosphorylated Mcm-2-7 hexamers with 200 microliters of high salt wash buffer. Mix by pipetting, ensuring the beads are fully resuspended.

Next, remove the buffer and wash the beads once with 200 microliters of CMG buffer. To assemble and activate the fluorescently-labeled CMG onto bead-bound DNA, remove the CMG buffer and resuspend the DNA-bound beads in 50 microliters of CMG buffer supplemented with five millimolar ADP. Next, mix all the components mentioned on the screen in one tube and place it on ice.

Immediately add the resuspended bead-bound DNA to the protein mix. Incubate the reaction for 15 minutes at 30 degrees Celsius with 800 RPM agitation. Wash the bead sequentially with HSW buffer and CMG buffer.

After removing the buffer, resuspend the CMG containing DNA-bound magnetic beads in 200 microliters of elution buffer, then incubate at room temperature for one hour with 800 RPM agitation. Place the tube in a magnetic rack and allow the beads to be collected for five minutes. Carefully collect the supernatant containing the eluted DNA-CMG complexes without disrupting the settled beads and transfer it to a new tube.

To ensure that no beads remain in the solution, place the collected supernatant again in a magnetic rack and keep for another five minutes carefully. Collect the supernatant and transfer it to a new tube. Add 1, 400 microliters of CMG buffer to the 200 microliters of supernatant.

The sample is now ready for single molecule imaging.