KP3 absorbs three nucleotides from mixed site ("TACTAGAG"), so that the key has a 5 nucleotide spacer region (i.e. "TAGAG") between the transcription start site and the first nucleotide of the key.
We are still looking into what the optimal size for the spacer region is. See also KP2 (BBa_J01007), and Key1 (BBa_J01008), Key2 (BBa_J01009), Lock1 (BBa_J01010), and Lock2 (BBa_J01011).