FIGURE 2.

Nucleotide specificity of transcription priming. A–C, short transcripts (16 or 17 nt) were made by L-A virions in a CTP-omitted transcription reaction. The transcripts were labeled with [α-³²P]UTP and primed without (None) or with 20 μm guanine nucleotides or their analogues as indicated above the panels. The products were separated in a 15% acrylamide gel and visualized by autoradiography. Some of the products were pretreated, before electrophoresis, with RNA 5′-polyphosphatase (RPPase), tobacco acid pyrophosphatase (PyroPase), or Terminator 5′-exonuclease (Ter Ex) as indicated below the panels. It should be noticed that the cap analogue (m⁷GpppG)-primed transcripts migrate as 17-mer because of an extra G at the 5′-end (A, lane 7). Its size can be reduced to 16-mer by tobacco acid pyrophosphatase treatment (B, lane 7). D, the nucleotide sequence of the L-A positive strand at the 5′-end region is shown. The asterisks indicate the positions in the transcripts, which can be labeled with [α-³²P]UTP in vitro.