148. The E. 𝑐𝑜𝑙𝑖 strain 𝐵𝐿21(𝐷𝐸3) is used for expressing recombinant proteins whose genes are cloned in the 𝑝𝐸𝑇 series vectors. The primary reason for this is:
1. 𝐵𝐿21(𝐷𝐸3) lacks proteases that degrade polymerases required for the expression of genes in PET vectors.
2. 𝐵𝐿21(𝐷𝐸3) encodes proteases that specifically process the expressed protein.
3. 𝐵𝐿21(𝐷𝐸3) harbors a polymerase that is required for expressing the genes in pET vectors.
4. 𝐵𝐿21(𝐷𝐸3) has more efficient protein translation system than cloning strains of E. 𝑐𝑜𝑙𝑖.

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Introduction

In recombinant protein production, choosing the right host strain is critical for efficient expression. E. coli BL21(DE3) is one of the most commonly used strains, particularly with the pET series vectors. The reason lies in its unique genetic composition that enables high-level expression of target proteins.

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Understanding the pET Vector System

The pET (plasmid for expression by T7 RNA polymerase) system is a powerful and tightly regulated system used to express genes cloned downstream of a T7 promoter. However, T7 promoters are not recognized by the native E. coli RNA polymerase.

This is where BL21(DE3) comes in.

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Why BL21(DE3) Is Used

The primary reason for using BL21(DE3) with pET vectors:

BL21(DE3) harbors the gene for T7 RNA polymerase under the control of the lacUV5 promoter on its genome.

Upon induction with IPTG, the lacUV5 promoter drives expression of T7 RNA polymerase, which then recognizes the T7 promoter on the pET plasmid, initiating transcription of the target gene.

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Evaluating the Answer Options

1. ❌ BL21(DE3) lacks proteases that degrade polymerases required for the expression of genes in PET vectors.
   Incorrect. While BL21 strains are deficient in certain proteases (e.g., Lon, OmpT), that’s not the primary reason it is used with pET vectors.

2. ❌ BL21(DE3) encodes proteases that specifically process the expressed protein.
   Incorrect. The strain is used to avoid proteolytic degradation, not to process proteins.

3. ✅ BL21(DE3) harbors a polymerase that is required for expressing the genes in pET vectors.
   Correct. It contains the T7 RNA polymerase gene, essential for transcribing genes under the control of a T7 promoter in pET vectors.

4. ❌ BL21(DE3) has more efficient protein translation system than cloning strains of E. coli.
   Partially true, but not the main reason for its use with pET vectors.

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Conclusion

The success of pET vector-driven expression relies on the presence of T7 RNA polymerase, which is encoded in the genome of E. coli BL21(DE3). This makes it the preferred host for high-level expression of recombinant proteins using pET plasmids.

Correct Answer:
3. BL21(DE3) harbors a polymerase that is required for expressing the genes in pET vectors.