The mol. wt. of the single-stranded RNA molecules synthesized in a cell-free system on simian virus 40 (SV40) DNA by RNA polymerase was determined at intervals after the initiation of the reaction. Chain elongation of the RNA, under the experimental conditions used, took place at an average rate of 5 nucleotides/s. Thus single-stranded RNA molecules of 1.5 × 10 mol. wt. resulting from continuous transcription of one turn of the circular SV40 genome did not appear until about 20 min after initiation of the reaction. After 40 to 80 min, molecules with mol. wt. equal to or greater than the mol. wt. of the DNA template strand (⩾ 1.5 × 10) accounted for 25 to 50% of the mass of the RNA population. Up to 25% of the RNA synthesized at that time had mol. wt. equal to or larger than 3 × 10. The results indicate that the cell-free system used does not contain specific factors or signals which would cause the RNA polymerase to terminate transcription.


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