Selective and accurate transcription of the Xenopus laevis 5S RNA genes in isolated chromatin by purified RNA polymerase III.

Chromatin isolated from immature oocytes was found to contain an endogenous RNA polymerase activity (RNA nucleotidyltransferase; nucleoside triphosphate:RNA nucleotidyltransferase, EC 2.7.7.6) that synthesizes predominately 5S RNA. However, the levels of total RNA synthesis and 5S RNA synthesis in chromatin were each stimulated 10- to 50-fold by an exogenous RNA polymerase III purified from X. laevis oocytes. The 5S genes in chromatin were transcribed by the exogenous enzyme in a highly selective (3000-fold above random) and predominately asymmetric fashion. A significant fraction of 5S RNA sequences were also found in a discrete transcript, approximately 5S in size. Total RNA synthesis was significantly stimulated when chromatin was transcribed by oocyte RNA polymerase I, murine RNA polymerase II, and low levels of Escherichia coli RNA polymerase. However, these enzymes did not significantly stimulate 5S RNA synthesis above the endogenous levels. Both homologous oocyte RNA polymerase I and III and E. coli RNA polymerase transcribed the 5S genes in deproteinized DNA to approximately the same extent (severalfold above random) and both the sense and anti-sense strands of the gene were transcribed. It appears, therefore, that both chromatin-associated components and a purified RNA polymerase III are necessary and sufficient for the selective and accurate transcription of the 5S RNA genes in vitro.