Difficulties in expression of protein in E. coli

[studer]

To my knowledge protein toxicity in the cell can cause the cell to excrete
the plasmid.  It is just a natural reaction to something that is killing
you.  The cells can be somewhat stable if they are make into frozen stock
and kept at -80C but some vectors (I have read) will even then exclude the
plasmid.  How are you storing the cells and have you tried to retransform
the plasmid.

sudhanshu vrati wrote:

> We are using a T7 promoter based expression vector in BL21 cells for
> expressing a viral protein.  The protein was successfully expressed,
> purified and sequenced to find it was the correct one.  But now the same
> vector in same cells refuses to make the desired protein.  Any clues ?
>
> Sudhanshu