In my experiment I found that a GFP gene under T7 promotor in high copy number pTAC-2 vector is showing green fluorescence producing colonies of DH5 alpha under UV light. what could be the possible reason?

  • $\begingroup$ Shouldn't pTAC have the tac promoter and not T7? $\endgroup$
    – canadianer
    Commented Sep 1, 2015 at 2:50
  • $\begingroup$ You got a T7 phage contamination? $\endgroup$
    – user137
    Commented Sep 1, 2015 at 6:39
  • 1
    $\begingroup$ Did you eliminate autofluorescent activities? $\endgroup$
    – 243
    Commented Sep 1, 2015 at 16:43
  • $\begingroup$ Confirm your strain has DE3 or other mutated lacp? Best practice to include selection antibiotic for the lac cassette as well. Also @243 answer above. Finally Check agar for any supplements and run map on plasmid to confirm it is ptac $\endgroup$
    – rhill45
    Commented Sep 1, 2015 at 20:55

1 Answer 1


Consider the phenomenon of zygotic induction, as originally observed by Art Pardee, Francois Jacob, and Jacques Monod (sometimes called the "PaJaMo", or "PaJaMa" experiment): regulation of a repressed promoter can be affected by the copy number of the promoter in the cell. Put another way, if the copy number of the lac operator in the cell is high enough to bind or titrate all of the available lac repressor protein in the cell, then some of the lac promoters in the cell will be expressed.


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