Sometimes in synthetic biology, we need to know rates of transcription of one promoter in relation to others (particularly inducible vs constitutive) in order to perform tasks like balancing transcription products and ensuring an excess of one product over another. As far as I know, there is no standard method of characterizing promoter strengths in relation to one another. However, a logical unit for transcription rate made (by organism or in vitro) could look something like:
Promoter Strength = number of transcript made / (concentration of RNA polymerase) (length of transcript) (unit of time).
Why hasn't this been done yet? What other factors could contribute to rate of expression in a particular organism? Is there another system for comparing promoter strengths?
Note: a similar system could be used for strength of repression.