why is it that only E.coli BL21(DE3) strain is widely used for eukaryotic gene expression when other strains can also be used. what is the full form of BL21(DE3). I searched but i did not find the full form. I already know that it is required in protein expression due to T7 polymerase.
The information in your post is not entirely accurate. First, the genotype can be found here:
http://openwetware.org/wiki/E._coli_genotypes#BL21.28DE3.29
Second, the DE3 refers to the fact that this strain is a lambda-phage lysogen, rendering it capable of expressing T7 (not T4) phage polymerase upon IPTG addition.
BL21 strains are useful for recombinant expression because they lack the lon protease and are less likely to degrade the recombinant protein. However, they are mainly used in T7-based expression, such as the pET vectors, that enable very low gene expression in the absence of IPTG induction. For other systems, such as pGEX where the GOI is directly under control of the lac promoter, you don't need the DE3 form, and while BL21 is recommended, it is not required.
Like what Daniel said, DE3 is for T7-based expression.
If you are expressing eukaryotic genes, I would recommend Rosetta2(DE3). It's similar to BL21(DE3), but it also supply tRNAs for rare codons (i.e. rare in E. coli, but more common in other organisms). Of course, it's best to try out different strains and compare the yield empirically.
First of all this strain like many B strain lacks Lon protease (cytoplasm) and OmpT protease (outer membrane) and hence it is preferred for eukaryotic gene expression. Secondly The DE3 designation means that respective strains contain the λDE3 lysogen that carries the gene for T7 RNA polymerase under control of the lacUV5 promoter. However,there are instances where other strains (as pointed out by Arthur) like Rosetta 2(DE3) is also used.
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