title: Activation of a mammalian origin of replication by chromosomal rearrangement.
abstract: The methotrexate-resistant Chinese hamster cell line DC3F/A3-4K (A3/4K) contains at least two prominent dihydrofolate reductase amplicon types. The type I amplicons, constituting approximately 80% of the total, are at least 650 kb in length, but the endpoints have not yet been characterized. The type II sequences represent approximately 20% of amplicons, are 450 kb in length, and are arranged as alternating head-to-head and tail-to-tail repeats. In previous studies on the CHOC 400 line, in which the amplicons are much smaller, a replication initiation locus (ori-beta/ori-gamma) has been shown to reside downstream from the dihydrofolate reductase gene. In a more recent study on the larger amplicons of A3/4K cells, we detected an additional initiation locus (ori-alpha) lying approximately 240 kb upstream from ori-beta/ori-gamma. Interestingly, in vivo labelling experiments suggested that replication forks diverge from ori-alpha only in the downstream direction. This finding suggested either that ori-alpha is a unidirectional origin or that a terminus lies immediately upstream from ori-alpha. However, in this study, we show that ori-alpha is actually very close to the head-to-head palindromic junction sequence between the minor type II amplicons in A3/4K cells; furthermore, ori-alpha is active in the early S period in the type II amplicons but not in the larger type I sequences that lack this palindromic junction. This is the first direct demonstration in mammalian cells that a cryptic origin can be activated by chromosomal rearrangement, presumably by deleting negative regulatory elements or by creating a more favorable chromosomal milieu for initiation.
authors.affiliation: Department of Biochemistry, University of Virginia School of Medicine, Charlottesville 22908.
authors.forename: T H
authors.initials: TH
authors.lastname: Leu
authors.forename: J L
authors.initials: JL
authors.lastname: Hamlin
mesh_terms: D000818:Animals; D002460:Cell Line; D006224:Cricetinae; D003412:Cricetulus; D004261:DNA Replication; D005784:Gene Amplification; D015321:Gene Rearrangement; D066298:In Vitro Techniques; D012045:Regulatory Sequences, Nucleic Acid; D015183:Restriction Mapping; D013762:Tetrahydrofolate Dehydrogenase / Q000235:genetics*
chemical_list: D013762:Tetrahydrofolate Dehydrogenase