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Template specificities of Xenopus laevis RNA polymerases. Selective transcription of ribosomal cistrons by RNA polymerase A.

1 
The activities of DNA-dependent RNA polymerases A and B purified from ovaries of Xenopus laevis were examined on various templates. 
 
2 
When bulk Xenopus DNA was centrifuged in caesium chloride gradients the ratio of incorporation of UMP and GMP into RNA by the two enzymes different in the region where the ribosomal cistrons banded when the fractionated DNAs were used as templates. The A enzyme selectively synthesised G-rich RNA under conditions where the enzyme :rDNA ratio was high, whereas no conditions were found under which the B enzyme would synthesise G-rich RNA. 
 
3 
Molecular hybridisation of the cRNA synthesised by A and B enzymes from DNA enriched with ribosomal cistrons and main-band DNA demonstrated that only transcripts of rDNA by the form A RNA polymerase were competed out by unlabelled 18-S and 28-S Xenopus ribosomal RNAs. 
 
4 
Both forms A and B RNA polymerases were capable of forming rifamycin AF/0-13 resistant complexes on either main-band or ribosomal DNA. However, whereas the amount of resistance by the B enzyme was similar on both templates, RNA polymerase A became much more resistant on rDNA than on main-band DNA. 
 
5 
Sensitivity to actinomycin D and the exotoxin from Bacillus thuringiensis, both of which inhibit ribosomal RNA synthesis preferentially in vivo, was apparently independent of the type of template used in these experiments.