Various parameters of transcription of cloned mouse rDNA have been examined using extracts from control P1798 cells and from cells treated 24h with 0.1 μM dexamethasone. Highly purified RNA polymerase I from either source catalyzes nucleotidyl transfer (elongation) at a rate of approximately 30 nucleotides/sec. Extracts from hormone-treated cells are capable of forming stable, preinitiation complexes. The rates of stable complex formation are the same in extracts from control and hormone-treated cells. Nevertheless, initiation of transcription does not occur in extracts from hormone-treated cells. Initiation in such extracts may be restored by the addition of a partially purified RNA polymerase I initiation factor, designated TFIC. The data indicate that initiation by RNA polymerase I is a multi-step process. The first step involves the formation of stable, preinitiation complexes, as demonstrated by a number of groups. Initiation, per se, requires an additional protein, TFIC. Glucocorticoids and perhaps other mitogenic agents regulate transcription of rDNA by influencing the amount or activity of TFIC.
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