pLysS freezer stock instability

Frank van de Loo fvandelo at
Mon Mar 13 11:04:35 EST 1995

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
(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
Do others observe the same phenomenon?  Is there a solution?  Are the 
BLR (RecA-) cells better?
Frank van de Loo.

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