Fragments of SP6 DNA generated by cleavage with Hpa II or Taq I were cloned into the Cla I site of pBR322 and the recombinant plasmids were screened for the presence of SP6 promoter activity by transcription in vitro with purified SP6 RNA polymerase. Three plasmids having promoter activity and small inserts of SP6 DNA were characterized. Hybridization studies showed that all three cloned promoters arose from different regions of the SP6 genome. Comparison of the consensus promoter sequence (5' ATTTAGGtgGACACTATAGAAGgaG 3') with the consensus sequences of promoters recognized by the T3 and T7 RNA polymerases reveals a common core sequence (5'-CACTA-3') extending from -7 to -3, as well as other features that may be important in selective promoter recognition by the phage RNA polymerases.

The early genes of bacteriophages T7 and T3 are transcribed by E. coli RNA polymerase from a cluster of three promoters that lie near the left end of the genome (1 ,2). We have sequenced the early control region of T3 DNA and have compared it with the corresponding region of T7 DNA (Fig. 1). Three potential promoters for the E. coli RNA polymerase (designated Al, A2 and A3) are apparent from their homology to the Al, A2 and A3 promoters of T7, as well as by their homology to the consensus sequence for other E. coli promoters (4).