PET Vectors expressing constitutively

ME. Homer bimpt at
Mon Oct 9 09:54:37 EST 1995

	We have a PET vector expressing a non-lethal protein. We have
transformed BL21(DE3) and XL1B strains of E.coli. The BL21 express with
IPTG induction (as expected). The XL1B express constitutively (giving
greater levels of expression than the induced BL21), with IPTG having
little, if any, effect on the level of expression.

	The expressed protein is of the correct size, the His-Tag at the 
N-terminus is cleavable to give a protein of the correct size and the
first ten residues of the cleaved protein have the correct sequence.

	XL1B do not contain a genomic copy of an inducible promoter or
of the T7 RNA polymerase gene required for expression so should not be
capable of of expressing the protein.

	Can anyone provide an explanation for (or, has anyone had
experience of) PET vectors expressing (constitutively or inducibly) in
what should be a non-expressing bacterial strain?

Mark Thomas.

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