pLysS freezer stock instability

Richard_Heath heath at
Tue Mar 14 14:58:31 EST 1995

In article <3k1qej$4ia at>, Frank van de Loo <fvandelo at> writes:
> Dear methodists,
> I find that my BL21(DE3)pLysS competent cells, and pET-containing
> strains derived from them, lose their chloramphenicol resistance
> during storage at -80.  The competent cells were grown in 
> chloramphenicol and stored with DMF; they also lose viability

Some strains will store better if they have not been grown in antibiotic, or at
least the antibiotic is removed (pellet cells, resuspend in fresh medium) 
before storage.  It seems that the ab can still "kill" the cells even at this
temp .... or maybe interferes with their recovery on subsequent plating.

> (very low after 1 year).  The pET-containing strains suffer dramatic
> decreases in IPTG-inducible protein expression over the same time
> frame.  Loss of pLysS alone should not cause reduced expression; in
> fact the same pET-based plasmid in BL21(DE3) still gives monstrous
> expression.
> Do others observe the same phenomenon?  Is there a solution?  Are the 
> BLR (RecA-) cells better?
> Thanks,
> Frank van de Loo.

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