about pET vector!

[Boss]

Hello, 
 
I've tried some expression experiments using a pET28 vector, to express a certain enzyme. And in my operation, I've modified the vector by adding a leader sequence to it, because I want to locate my protein in periplasm. Now I found the location failed. Furthermore, the expression level had been quite low. My protein only occupied about 8% of the total proteins.
Here's my trouble. would someone please give me some advice?
Thanks a lot.
 
Chen
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