E coli strain JM109(DE3) has its genotype listed as endA1 glnV44 thi-1 relA1 gyrA96 recA1 mcrB+ Δ(lac-proAB) e14- [F' traD36 proAB+ lacIq lacZΔM15] hsdR17(rK-mK+) (λDE3).
It seems like it should be suitable for blue-white screening because of the lacZΔM15 deletion and but Promega states it isn't suitable. Another supplier, IntactGenomics says it is good for blue-white screening.
Anyone have an explanation? Is it just that it's an expression strain and not a cloning strain?
Thanks!
Probably need to add IPTG for blue/white screening. Many cloning strains lack LacI so can do blue/white screening on plates with just X-GAL.
I agree with Robert Adolf Brinzer that JM109 is (in theory) suitable for blue white screening. I have used the non-DE3 version often in the past. However it can lose the F' which carries the lacIq lacZΔM15 fairly easily so you may wish to steak the strain on minimal medium without any amino acids. The F' is maintained because it carries the proAB genes to complement the proAB deletion on the chromosome.
Another issue is that while you might need to add IPTG to really produce good color, in doing so you are doing to induce T7 polymerase expression from the DE3 prophage. So this may actually counter select against the colonies you want if you are cloning into a T7 expression system. So be cautious that you may be specifically selecting against the clones you want. That is probably why Promega says to not use it that way.
You would be better off to do the initial blue-white screening in a non-DE3 strain and then when you find the correct plasmid to retransform the DNA. However if you don't have any non-DE3 strains then you might try with just X-gal and no IPTG and you might still be able to see blue/white but more faintly.
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