Chromosome structure and DNA sequence alterations associated with mutation of transformed genes

We have constructed a series of tk+ cell lines by DNA-mediated gene transfer to correlate chromosomal behavior and DNA sequence alterations associated with reversion to the tk- phenotype. Tk- revertants were selected from each of four well-characterized transformed cell lines containing the viral tk gene and multiple human growth hormone genes (HGH). Tk- colonies were analyzed for the presence of tk and HGH sequences by blot hybridization to restriction endonuclease cleaved DNA. Revertants were further characterized by detailed karyotype analysis and hybridization in situ. Blot hybridization of forty tk- revertants indicates that over half of the revertants delete all of the transforming DNA from the recipient chromosome. In fifteen additional revertants, significant deletion has occurred, although transforming DNA is retained. The analysis of chromosomes by Giemsa banding together with hybridization in situ reveals that the deletion of transforming DNA is never associated with loss of an entire chromosome. Reversion to the tk- phenotype, therefore, seems to involve discrete deletions of transforming DNA without apparent chromosome loss. In this restricted set of mutants, it thus seems crucial to maintain the diploid chromosomal complement.