Presence of an inhibitor of RNA polymerase I mediated transcription in extracts from growth arrested mouse cells.

Extracts obtained from mouse cells growth arrested at stationary phase or under serum starvation exhibit no specific rDNA transcription activity. Experiments with mixed transcriptionally active and inactive whole cell extracts (WCE) obtained from rapidly dividing or growth arrested cells, respectively, demonstrate that rRNA synthesis in vitro can be suppressed by a polymerase I transcription inhibitory activity (PIN), present in inactive extracts. This inhibition effect is not related to increased nuclease activity and affects neither the non-specific Pol I transcription, nor a polymerase II promoter. A comparison of WCE isolated under different growth conditions indicates that PIN changes according to the physiological state of the cell. It reaches a maximal level soon after serum depletion and disappears rapidly when cells are allowed to recover in serum-rich medium. PIN can be clearly demonstrated in WCE but not in nuclear or cytoplasmic extracts and can be also obtained by an additional high salt extraction of nuclei. Furthermore, gel retardation and transcription-in-pellet assays demonstrate that rDNA promoter binding and preinitiation complex stability are similar in active and inactive WCE. This indicates that some later stage(s) of rDNA transcription, rather than the preinitiation complex formation, are attenuated by inactive extracts. Analysis of partially fractionated extracts suggests that PIN is not associated with but can be separated from polymerase I.