Chromosomal Mcm2-7 distribution is a primary driver of genome replication timing in budding yeast, fission yeast and mouse

The spatio-temporal program of genome replication across eukaryotes is thought to be driven both by the uneven loading of pre-replication complexes (pre-RCs) across the genome at the onset of S-phase, and by differences in the timing of activation of these complexes during S-phase. To determine the degree to which distribution of pre-RC loading alone could account for chromosomal replication patterns, we mapped the binding sites of the Mcm2-7 helicase complex (MCM) in budding yeast, fission yeast, mouse and humans. We observed identical MCM double-hexamer footprints across the species, but notable differences in their distribution: In budding yeast, complexes were present in sharp peaks comprised largely of single double-hexamers; in fission yeast, corresponding peaks typically contained 4 to 8 double-hexamers, were more disperse, and showed a striking correlation with AT content. In mouse and humans, complexes were even more disperse, with a preference for regions of high GC content. Nonetheless, most fluctuations in replication timing in all four organisms could be accounted for by differences in chromosomal MCM distribution. This analysis also identified genomic regions whose replication timing was clearly not attributable to MCM density. The most notable was the inactive X-chromosome, which replicates late in S phase despite the fact that both MCM abundance and chromosomal distribution were comparable to those on the early replicating active X-chromosome. We conclude that, although certain genomic regions, most notably the inactive X-chromosome, are subject to post-licensing regulation, most differences in replication timing along the chromosome reflect uneven chromosomal distribution of stochastically firing pre-replication complexes.