Hi there,

The phenomenon you describe is actually the leaky expression of the target protein which results in fact that the promoter controlling T7 RNA polymerase expression in not 100% locked. Therefore the leaky expression of the polymerase allows a basal expression or YFP (which sometimes is enough). If you want to lock the system more tightly you have to transform the recipient strain with pLysS plasmid (which produces lysozyme which inhibits the leaky activity of T7 RNA polymerase).

Dear Mauro Jesús Reyes Rosas

which kind of expression vector and promoter did you used in BL21SI? Did you use vectors based on T7 promoter or other (as TAC) that do not need the T7 polimerase?

Which is the salt concentration in your culture media?

I'm not an expert of it (i used in the past the BL21 AI but never SI) but similarly to the AI it seems that this strain guarantee a sort of double control of the induction since the expression of T7 polimerase is under the regulation of the salt inducible promoter and therefore theoretically this has to minimize the protein expression in the absence of the inductor for plasmid based on T7 promoter and if this will not happen with other promoters that works with E.coli polimerases or with culture medium those contain high salt concentration.

best regards

Manuele

As Dominique wrote, this is leaky expression. In accordance to this

http://cdfd.org.in/pdfs/lbg/BL21-SIpaper.pdf

they got about 10-times higher expression with 200 mM NaCl than with nothing.

If you need to get no expression without induction, you'll need to find other expression system.

Also, what medium did you use? Was it complex or minimal medium?