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

In some vertebrates tandem repeats (TTAGGG)_n are located not only in telomeres, but also in intrachromosomal sites. In Chinese hamster cells such interstitial repeats which may be called "telomeric" heterochromatin (THC), representing up to 5% of genome. Earlier we have shown, that blocks of THC dynamically bind telomeric protein TRF1 in Chinese hamster cells. In this work question has been studied whether this interaction depends on a transcription. In cells with the normal transcription around 85% of initial fluorescence intensity of GFP-TRF1 is restored in 60 sec after the photobleaching. Treatment of the cells with transcription inhibitor actinomycin D (ActD) in the concentration completely inhibiting activity of DNA-dependent RNA polymerases I and II (RPI and RPII) leads to fast and practically full suppression of exchange GFP-TRF1 (10% of initial fluorescence is restored only) whereas an inhibitor of protein synthesis cycloheximide (CHD) has not effect. At the low ActD concentration, suppressing only RPI, efficiency of recovery of fluorescence was not changed. Since some fractions of heterochromatin in mammalian cells are actively transcribed, exchange of GFP-TRF1 can be connected to transcription of THC which may be necessary for synthesis of small interfering RNA and self-maintenance of the heterochromatin, or with inhibition of expression of other genes effecting TRF1 stability.