pIVEX and BL21(DE3)

[DOCTORHIM at aol.com]

One word of caution when using BL21(DE30) and a new plasmid.  Many plasmids 
contain the Lac operator sequence since they were derived from plasmids that 
used the LacZ complementation system for blue/white colony selection.  This 
sequence will titrate out the small amount of Lac repressor in the strain 
thus inducing the T7 polymerase.  The end result will be the inability to 
stably introduce your plasmid as the expression of the protein will kill the 
cells.  

---