Modulation of transfected gene expression mediated by changes in chromatin structure

We examined the molecular basis of modulation of transfected herpes simplex virus (HSV) thymidine kinase (tk) gene in mouse fibroblasts. We observed that one tk+ cell line is capable of reversion to a tk- phenotype, and rereverts to a tk+ phenotype at high frequencies (5%). The revertants lacked tk enzyme activity and tk-specific transcripts. We detected no differences in the organization of foreign DNA or in the CpG methylation patterns in the revertants and rerevertants. We probed the chromatin structure of the revertants and rerevertants, and found the tk sequences in the rerevertants to be more highly sensitive to digestion with DNAase I than the corresponding revertant cells. We conclude that the high frequency switching of gene expression we observed is mediated by changes in chromatin structure, and that this may reflect the behavior of the host sequences at the site of foreign DNA integration.