Evidence for the nuclear origin of RNA polymerases identified in the cytosol: Release of enzymes from the nuclei isolated in isotonic sucrose

Abstract

DNA-dependent RNA polymerases were extracted from rat liver nuclei isolated in isotonic or hypertonic sucrose and resolved chromatographically. Activities of RNA polymerases I, II and III from “hypertonic nuclei” were 3.8, 1.5 and 27-fold, respectively, higher than those from “isotonic nuclei”. RNA polymerase III was virtually absent in the “isotonic nuclei”. Concurrent with the reduced revovery of polymerases from the “isotonic nuclei”, the corresponding cytosol fraction contained higher levels of all three enzymes. These studies provide strong evidence for the contention that a large proportion of cytoplasmic RNA polymerase C previously identified in “isotonic cytosol” fraction corresponds to nuclear RNA polymerase III. These data demonstrate the need for the use of hypertonic sucrose in the initial homogenization of tissues in order to prevent leakage of the nuclear enzymes.