Modulation of tk expression in mouse pericentromeric heterochromatin.

We have obtained a mouse transformant cell line containing two herpes viral thymidine kinase (tk) genes integrated in pericentromeric heterochromatin. Restriction analysis of tk- revertant and tk+ rerevertant derivatives suggest that one of the two tk genes is repressed in tk- cells, but is reactivated in tk+ rerevertants. The results of Northern analysis indicated that repression-activation is probably controlled at the transcriptional level. To examine the molecular basis for this repression, we cloned the tk gene from a tk- revertant cell line. Then, using the cloned tk gene as donor DNA to select for tk+ transformants, we found that it has a transfection efficiency indistinguishable from the viral tk gene. This indicates that repression is probably not mediated via any DNA sequence changes within the tk gene. The results of further studies by restriction analysis, azacytidine treatments, and secondary DNA transfection assays demonstrated that tk repression is associated with changes in DNA methylation. Surprisingly, derepression of the tk gene was accompanied by rearrangements in the flanking DNA. The latter result suggests that the flanking DNA may exert cis effects on tk gene expression. Additional studies with this system may provide insights into the molecular basis underlying position effects in heterochromatin.