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Can a gene be expressed under the T7 promoter in an E. coli strain (e.g. DH5 alpha), which does not have the T7 polymerase gene encoded in its genome? In other words, is T7 promoter leaky?

To be more specific, how is it possible that a regular E. coli strain, which does not encode for the T7 polymerase, can grow on kan selective media if it was transformed with a plasmid that has the kanR gene under T7 promoter?

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I know I'm digging this up, mostly b/c I'm running into some T7 issues, but if you are referring to a specific experience, are you sure plasmid has the kan resistance under T7 promotion, not just the insert? –  Atl LED Oct 26 '13 at 2:21
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Apparently not, leakiness can be controlled by tightly regulating the T7 polymerase with a tight promoter (in this case lacUV5).

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Isn't it possible that a regular E. coli strain that does not have T7 polymerase, has other polymerases that can recognize the T7 promoter? –  Gergana Vandova Jun 29 '12 at 16:09
    
The reference points to BL21. I took a look at the MG1655 genome to see how many copies of the alpha I or II subunit, which with sigma subunits, defines the sequence specific binding affinity of the native polymerase. That should reveal any variations in the RNA polymerase binding. There is only a single copy of RNA polymerase alpha (which along wg of RNA Polymerase). Its hard to say that something like this is impossible. Its true that DH5alpha isn't a B or K12 derivative I think, so its always possible, but there is not rnap component mentioned in the strain description. –  shigeta Jun 30 '12 at 3:00
    
@GerganaVandova I've been thinking about this, and it seems to me that phage gene transfers might be relatively rare in bacteria because their genomes have so much genetic flux. a new polymerase would not affect any of the native genes - it would require quite a few mutations for a second polymerase to take hold in the genome given how trim bacteria tend to make themselves. –  shigeta Jun 30 '12 at 14:39
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