DH5a vs XL1Blue

[Benoit Dérijard]

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

[Arnoud van Vliet]

> 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
cheaper.

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
Arnoud

[Rudi van de Wetering]

> 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.

Thougth you also had to add Maltose for selection/expression of the F' (at
least if you use LB). And that the F' is only important if you're gonna
infect them with a bacteriophage. Anyway, that is the only thing i use
XL-blue for.

[Dr. Duncan Clark]

In article <90810a$8fb$1...@wnnews.sci.kun.nl>, the eminent Rudi van de
Wetering at University of Nijmegen, The Netherlands wrote

>Thougth you also had to add Maltose for selection/expression of the F' (at
>least if you use LB).

Maltose will have no effect on selection of the F'. What maltose will do
is repress lac expression even more. The F' in XL1Blue is Tetracycline
resistant so selection with Tet is all one needs to maintain the F'.

Duncan
--
The problem with being on the cutting edge is that you occasionally get
sliced from time to time....

Duncan Clark
GeneSys Ltd.
Tel: +44(0)1252376288
FAX: +44(0)8701640382
http://www.dnamp.com
http://www.genesys.demon.co.uk

[Alistair Forrest]

I didn't know about the lacIq thing and the XL1 blues giving tighter
repression, however in my experience i've found XL1-blue to give a higher
success rate in maxi-preps when compared to DH5alphas.

Take the same plasmid, tansform into XL1-blues and DH5a and do a minprep
->DNA OK
take some of the miniculture and seed maxi
XL1-blue -> DNA most of the time
DH5alpha -> DNA less of the time (bugs spat out plasmid??)

The XL1 blue's are much slower growing, tighter control?

For larger plasmids I use STBL2's, My XL1-blues are much more competent,
but the STBL's are basically "never fails" when it comes to maxi's. When
the XL1 blues work for maxi's they generally give higher yields than the
STBL2's, but the STBL2's work almost all the time and give a yield
approximately 70% that of the XL1-blues.

cheers,
Al