A Novel Function of Poly(ADP-ribosyl)ation:  Silencing of RNA Polymerase II-Dependent Transcription

Poly(ADP-ribosyl) transferase (ADPRT) is a nuclear enzyme that catalyzes the synthesis of ADP-ribose polymers from NAD+ as well as the transfer of these polymers onto acceptor proteins. The predominant acceptor of the poly(ADP-ribose) chains appears to be the enzyme itself. The function of ADPRT is thought to be related to a number of nuclear processes, including DNA repair and transcription. In this study, it was found that polymerase II-dependent transcription in nuclear HeLa extracts was repressed in the presence of NAD+ at concentrations as low as 1 microM. This repression was strictly dependent on the activity of ADPRT and correlated with the auto(ADP-ribosyl)ation of the enzyme. Subsequent degradation of the ADP-ribose polymers by enzymatic activities present in the nuclear extracts restored transcriptional activity. It would appear from these results that poly(ADP-ribosyl)ation represents the key event of the mechanism underlying NAD(+)-dependent silencing of transcription. Importantly, ADPRT- and NAD(+)-dependent silencing was observed only if poly(ADP-ribosyl)ation had taken place before formation of the transcription complex was completed. That is, if the nuclear extract was preincubated for more than 15 min in the presence of template DNA, transcription was rendered entirely insensitive to NAD+. These results suggest that poly(ADP-ribosyl)ation may prevent polymerase II-dependent transcription, but does not interfere with ongoing transcription. Taking into account the known function of ADPRT, this enzyme may facilitate recovery from DNA damage by stimulating DNA repair and silencing transcription.