Molecular analysis of chromosome-mediated gene transfer

Metaphase chromosomes isolated from a cell line carrying the thymidine kinase (TK) gene of herpes simplex virus type I were used to transform the TK-deficient cell line LMTK- to the TK+ phenotype. Four independent transformants were isolated, all of which expressed virus-specific TK. Each of the four transformant cell lines initially became TK- at a rate of 12% per day. All four transformants possessed multiple copies of the TK gene and in one of the four a rearrangement occurred adjacent to the TK sequences. Stable TK+ derivatives of each line, isolated after prolonged cultivation, retained fewer copies of the TK gene than did their unstable parents. The transferred chromosomal fragment was larger than 17 kilobases in each line.