Dynamic binding of the human telomeric protein TRF1 to intrachromosomal blocks (TTAGGG)n in live Chinese hamster cells found to depend on transcription

Chromosomes of some vertebrates contain (TTAGGG)_n tandem repeats not only in telomeres, but also in internal, often pericentric, regions. Such internal repeats, termed telomeric heterochromatin (THC), account for 5% of the genome in Chinese hamster. THC blocks dynamically bind the telomeric protein TRF1. The purpose of this study was to find out whether or not this interaction depends on transcription. Cells with normal transcription recovered approximately 85% of the initial fluorescence intensity (FI) of GFP-TRF1 60 s after photobleaching. The transcription inhibitor actinomycin D (ActD) quickly and almost completely suppressed GFP-TRF1 exchange (only 10% of the initial FI was recovered) when used at a concentration fully inhibiting DNA-dependent RNA polymerases I and II. The protein synthesis inhibitor cycloheximide had no such effect. The efficiency of FI recovery did not appreciably change when ActD was used at a low concentration to suppress only RNA polymerase I. Since some fractions of heterochromatin are actively transcribed in mammalian cells, it was assumed that GFP-TRF1 exchange is associated either with heterochromatin transcription, which is possibly necessary for siRNA synthesis and self-maintenance of heterochromatin, or with inhibited expression of other genes affecting the TRF1 stability.