RNA synthesized in vitro by calf thymus RNA polymerase III (C), as well as by E. coli RNA polymerase, is restricted to a subset of calf thymus DNA.

RNA synthesized in vitro from chromatin and DNA by calf thymus RNA polymerase III was evaluated by hybridization in vast DNA excess. The RNA contains RNA complementary to both moderately repeated and unique DNA sequences. Very highly repeated DNA is not transcribed. A greater portion of RNA transcribed from DNA by RNA polymerase III hybridizes to moderately repeated DNA than RNA transcribed by Escherichia coli RNA polymerase. In studies utilizing DNA absorbed to filters, RNA transcribed from chromatin in short incubations hybridized to a greater extent than RNA transcribed for longer times. Similar results were obtained with RNA transcribed from DNA by E. coli RNA polymerase. These results suggest: 1) RNA polymerase III may be responsible for the synthesis of RNA species in addition to tRNA and 5 S ribosomal RNA and a portion of this RNA is transcribed from unique DNA; and 2) in vitro there may be selectivity in the initiation of transcription by both E. coli RNA polymerase and calf thymus RNA polymerase III.