Lagging-strand, early-labelling, and two-dimensional gel assays suggest multiple potential initiation sites in the Chinese hamster dihydrofolate reductase origin.

There is general agreement that DNA synthesis in the single-copy and amplified dihydrofolate reductase (DHFR) loci of CHO cells initiates somewhere within the 55-kb spacer region between the DHFR and 2BE2121 genes. However, results of lagging-strand, early-labelling fragment hybridization (ELFH), and PCR-based nascent-strand abundance assays have been interpreted to suggest a very narrow zone of initiation centered at a single locus known as ori-beta, while two-dimensional (2-D) gel analyses suggest that initiation can occur at any of a large number of potential sites scattered throughout the intergenic region. The results of a leading-strand assay and two intrinsic labelling techniques are compatible with a broad initiation zone in which ori-beta and a second locus (ori-gamma) are somewhat preferred. To determine how these differing views are shaped by differences in experimental manipulations unrelated to the biology itself, we have applied the lagging-strand, ELFH, neutral-neutral, and/or neutral-alkaline 2-D gel assays to CHOC 400 cell populations synchronized and manipulated in the same way. In our experiments, the lagging-strand assay failed to identify a template strand switch at ori-beta; rather, we observed a gradual, undulating change in hybridization bias throughout the intergenic spacer, with hybridization to the two templates being approximately equal near a centered matrix attachment region. In the ELFH assay, all of the fragments in the 55-kb intergenic region were labelled in the first few minutes of the S phase, with the regions encompassing ori-beta and ori-gamma being somewhat preferred. Under the same conditions, neutral-neutral and neutral-alkaline 2-D gel analyses detected initiation sites at multiple locations in the intergenic spacer. Thus, the results of all existing replicon-mapping methods that have been applied to the amplified DHFR locus in CHOC 400 cells are consistent with a model in which two somewhat preferred subzones reside in a larger zone of multiple potential initiation sites in the intergenic region.