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 catalyz es the synthesis of ADP-ribose polymers from NAD(+) as well as the tra nsfer of these polymers onto acceptor proteins. The predominant accept or 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 pro cesses. including DNA repair and transcription. In this study, it was found that polymerase II-dependent transcription in nuclear HeLa extra cts was repressed in the presence of NAD(+) at concentrations as low a s 1 mu M. This repression was strictly dependent on the activity of AD PRT and correlated with the auto(ADP-ribosyl)ation of the enzyme. Subs equent 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 w as observed only if poly(ADP-ribosyl)ation had taken place before form ation of the transcription complex was completed. That is, if the nucl ear extract was preincubated for more than 15 min in the presence of t emplate DNA, transcription was rendered entirely insensitive to NAD(+) . These results suggest that poly(ADP-ribosyl)ation may prevent polyme rase II-dependent transcription, but does not interfere with ongoing t ranscription. Taking into account the known function of ADPRT, this en zyme may facilitate recovery from DNA damage by stimulating DNA repair and silencing transcription.