Cleavage of the 190-kDa subunit of DNA-dependent RNA polymerase I yields small polypeptides capable of degrading DNA.

The lability of RNA polymerase I has been examined by incubation at 37 degrees C. Under the conditions used virtually all enzyme activity was lost after 90 min incubation at this temperature. Concomitant with the decrease in enzyme activity was the disappearance of the 190-kDa enzyme subunit and the production of three small polypeptides of Mr 14,000-19,000. Using immunological methods the small polypeptides were shown to be cleavage products of the 190-kDa subunit. The 190-kDa-derived polypeptides retained all the antibody-binding sites of the large subunit that are important in RNA synthesis. The isolated cleavage products were able to bind to DNA as well as to active enzyme and to inhibit reversibly RNA synthesis. In addition to their binding properties, the 190-kDa-derived polypeptides, but not the intact 190-kDa subunit, were able to degrade DNA. These data suggest that multiple mechanisms exist in the control of RNA polymerase I-catalyzed transcription, namely, irreversible loss of the catalytic subunit coupled with reversible inhibition by the cleavage products as a result of competition for normal enzyme-DNA interactions and, possibly, involving activation of a cryptic nuclease.