The bacteriophage promoters, T7, T3, and SP6, consist of 23 basepairs numbered -17 to +6, where +1 indicates the first base of the coded transcript. An important observation is that, of the +1 through +6 bases, only the base composition of +1 and +2 are critical and must be a G and purine, respectively, to yield an efficient transcription template. In addition, synthetic oligonucleotide templates only need to be double-stranded in the -17 to -1 region of the promoter, and the coding region can be all single-stranded. This can reduce the cost of synthetic templates, since the coding region (i.e., from +1 on) can be left single-stranded, and the short oligonucleotides required to render the promoter region double-stranded can be used with multiple templates.
Figure 1. Consensus Promoter Sequences. The +1 base (in bold) is the first base incorporated into RNA during transcription. The underline indicates the minimum sequence required for efficient transcription.