ABSTRACT
The identification of the type I junction allowed us to establish the correct permutation of the circularly permuted type I map vis a vis the parental dihydrofolate reductase (DHFR) locus. The high copy number made it possible to visualize restriction fragments arising from the amplicons in digests separated on ethidium bromide-stained agarose gels. The earliest detectable DHFR amplicons were almost invariably located on chromosome arm as one of the parental, single copy loci. Initial recombinant clones from the amplicons in CHOC 400 cells were isolated from a cosmid library prepared in the vector pHC79. In quantitative hybridization studies, the type I sequence was shown to represent only 5–10% of the amplicons in CHOC 400 cells. In situ hybridization studies on the relatively early stages of DHFR gene amplification in Chinese hamster ovary cells also suggest that initial amplicons can be extremely large.
