DH5a vs XL1Blue

Arnoud van Vliet avvliet at knoware.nl
Thu Nov 30 17:37:35 EST 2000

> Does anybody knows the advantage of DH5alpha over XL1Blue or the opposite
> Txs for helping.

Xl1-Blue is a LacIq strain (and requires tetracycline for selection of the
F'), whereas DH5alpha is not. That means that XL1-blue overexpresses the
lacI protein, and thus represses the lacZalpha gene on plasmids like pUC19
much better than Dh5alpha.

That means that when you do cloning with blue-white screening in DH5alpha,
you don't have to add IPTG to the plate; if it is lac-positive it will stain
blue anyway. With Xl1-blue you have to add the IPTG, otherwise the
blue-white staining won't work properly. So working with DH5alpha is

But, on the downside, if your cloned fragment is encoding something toxic,
then it won't clone at all in DH5alpha because expression from the lac
promoter is not repressed, while in XL1-blue it is repressed. So then it is
better to use XL1-blue

My opinion: start with DH5alpha, and if your clonings don't work, switch to
a lacIq strain.

hope this helps

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