Relative promoter strength - (Sep/27/2011 )

Hi all, I had been transfecting 293ts using a plasmid with a CMV promoter and have recently changed to a different plasmid with a hEF1a promoter and I'm observing significant differences between these two plasmids. Do people know the relative strengths of promoters? From what I've seen I'd expect hEF1a to be a stronger promoter than CMV but am hoping to get some outside opinions on this.

Any other explanations people have for this would also be greatly appreciated.

Thanks

CMV is a viral promoter, and hEF1a is a human gene promoter so I would have expected the viral promoter to be stronger, but I don't really know.