Cell repair after liver injury: Stimulation of RNA synthesis, engaged polymerases, number of RNA transcribing molecules, and elongation rate in postischemic liver nuclei

Abstract

Nuclei isolated from liver cells recovering from reversible (nonnecrogenic) ischemia show an increased RNA synthesis which is essentially sustained by the enhanced activity of the “engaged” (endogenous template-dependent) polymerases; “free” (exogenous templatedependent) polymerases do not change in activity. Chromatin isolated from postischemic nuclei, assayed with Escherichia coli polymerase, shows a normal template activity. Under experimental conditions known to maximize elongation, such as the presence of heparin and high ionic strength, the difference between normal and postischemic nuclei persists, suggesting that stimulation of RNA synthesis during the postischemic period may be caused in part by an increase in the number of polymerase molecules engaged in transcription. But under the same conditions stimulation of RNA synthesis decreases, implying that changes in elongation rate may also play a role in the postischemic effect. Measurements of [ 3H]UMP, derived from internal nucleotides, and [ 3H]uridine, derived from 3′-termini, and subsequent calculation of polynucleotide elongation rate and number of transcribing polymerase molecules confirm that postischemic recovery enhances RNA synthesis mainly by increasing the number of transcribing polymerases and in addition by stimulating the elongation rate of the α-amanitin-resistant polymerase I+III forms.