What is the essential difference between two strains of E.coli: BL21(DE3) and TOP10? Why is BL21 used for protein expression and TOP10 for plasmid storage? The plasmid in this case is pDEST17. I'm really new at all this and looking at genome of these two doesn't give me any answers... I know it has something to to with lack of lac operon in TOP10 strain. Please help.

Bl21(DE3) is deficient in the Lon protease, thus reducing the chance of the degradation of the product to be overexpressed. Moreover, it carries an IPTG inducible T7 RNA polymerase. I.e. you can control the overexpression by cloning your gene downstream of a T7 promoter.