Transcription of the early region of bacteriophage T7: Selective initiation with dinucleotides

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A new picture of the in vitro transcription of the early region of bacteriophage T7 is presented. Dinucleotides were used to stimulate transcription by RNA polymerase from selected initiation sites on T7 DNA. Five initiation sites (three of which are very close together in the early promoter region) and five termination sites have been mapped relative to deletions in the early region. Over 20 r-strand specific RNA products arising from these sites have been characterized, most of which result from readthrough. Our results provide a strong correlation between in vivo and in vitro transcription of T7 by RNA polymerase. An additional initiation site outside the early region gives rise to a small l-strand specific RNA product. The dinucleotides have enabled us to determine whether ATP or GTP is used to initiate RNA chains at each initiation site. Since the dinucleotide primers apparently base-pair with the template at initiation sites, we have predicted short initiation site sequences, including the first few bases of the RNA chains.