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.
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