tac vs T7 promoter, which is stronger?

Dr. Peter Gegenheimer PGegen at UKans.edu
Fri Aug 15 19:09:09 EST 1997

On Fri, 15 Aug 1997 22:56:31, "Dr. Said Dermime" 
<sdermime at rpms.ac.uk> wrote:

> Dear All,
> Both tac and T7 are considered 'strong' promoters which are frequently
> used for expression of heterologous proteins in E. coli. Has anybody
> done quantitative work on the relative strength of these two using a
> reporter gene? If yes then could you point me towards the paper, 
> please.

Generally the T7 promoter will appear to be "stronger" in that 
you'll get much more protein made from a T7 overexpression system 
that from a tac-driven system. With the Studier T7 system (pET 
vector and chromosomally-located T7 RNAP) one can easily [i.e., 
there are many reports in the lit., and we get this level with most 
of the proteins we've tried] express the T7-driven gene to >= 50% of
total cell protein. I've never seen such levels from normal tac/trc 
promoters, but I don't know of a side-by-side comparison. 

Two caveats: first, a system with dual tac promoters gave excellent 
expression of glycogen phosphorylase (paper from Bob Fletterick's 
lab a year or so back), and because the rate of RNAP initiation can 
be fine-tuned by the concentration of IPTG, they were able to get 
low induction over a long period (1-2 days), so the protein remained
soluble. All of our T7-expressed proteins, with the notable 
exception of groE chaperonin(!), were made as inclusion bodies.

Second, the "strength" of a promoter ought properly to mean either 
the binding affinity for RNAP, or the clearance rate (rate at which 
bound RNAP "clears" the promoter, i.e., is converted from 
"initiating" to "elongating" polymerase). The former can be compared
only with the same RNAP; the latter could be compared among any 
promoter/polymerase combinations. But no, I don't have references at

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