Analysis of plant RNA polymerase I transcript in chromatin and nuclei

Chromatin isolated (pH 8.0) from soybean hypocotyl contains only RNA polymerase I activity as judged by its elution at low ionic strength (0.11 M ammonium sulfate) from DEAE-cellulose and DEAE-Sephadex, its total resistance to alpha-amanitin, and lack of preference for poly(dA-dT). The in vitro RNA product from this chromatin contains rRNA as a major component (36%) with little or no symmetry of transcription. The transcript from nuclei, where both RNA polymerases I and II are active, shows a dramatic increase in % rRNA (from 35 to 65%) when alpha-amanitin is present during synthesis. These observations suggest that plant RNA polymerase I is similar to animal RNA polymerase I in both its insensitivity to alpha-amanitin and preferential transcription of rRNA genes.