Mammalian Promoter Strength
What is the strongest mammalian promoter? Cytomegalovirus (CMV) appears to be the de facto standard, but how close is it to the true maximum promoter strength? An iGEM team from Heidelberg built a mammalian system for characterizing promoter strength. They show that CMV is a strong promoter relative to a baseline (JeT) but they do not get as far as developing a new constitutive synthetic promoter to replace CMV. The team does seem to think that there is room for improvement over CMV though.
Schlabach et al. had a paper in 2010 where they investigated all possible 10-mers as possible enhancers to a minimal CMV promoter:
We have taken a synthetic biology approach to the generation and screening of transcription factor binding sites for activity in human cells. All possible 10-mer DNA sequences were printed on microarrays as 100-mers containing 10 repeats of the same sequence in tandem, yielding an oligonucleotide library of 52,429 unique sequences. This library of potential enhancers was introduced into a retroviral vector and screened in multiple cell lines for the ability to activate GFP transcription from a minimal CMV promoter. [...]
None of the elements were capable of achieving the same levels of transcriptional enhancement across all tested cell lines as the CMV enhancer.
A stronger-than-CMV mammalian promoter would be extremely useful for many applications.