Cytogenetic analysis of amplification and deamplification of UMP synthase genes in Chinese hamster cells

Chinese hamster lung cells selected for resistance to pyrazofurin and 6-azauridine contain amplified UMP synthase genes. With selection in 5-fluorouracil, cells that have lost the amplified gene copies can be isolated. Reselection of deamplified cells in pyrazofurin and 6-azauridine results in reamplification of the UMP synthase genes. We have used chromosomal banding and in situ hybridization techniques to characterize this cyclic process of amplification and deamplification. Homogeneously staining regions (HSRs) were observed in cells containing amplified copies of the UMP synthase gene but not in cells in which the amplified UMP synthase genes had been lost. After reselection in pyrazofurin and 6-azauridine, abnormally banded regions (ABRs) were observed. Both HSRs and ABRs were located at a single site on the distal regions of a small acrocentric autosome, and both were shown to contain the amplified genes. The majority of 5-fluorouracil-selected cells showed residual marker acrocentric chromosomes of various sizes, suggesting excision of portions of the HSR or ABR as the mechanism of deamplification. The acrocentrics carrying the amplified genes resulted from rearrangements involving chromosome 4, site of the endogenous gene. This reversible selection system provides a unique model for investigating gene amplification and deamplification in association with chromosomal rearrangements and the relationship of G-banding to underlying DNA structure.