pBR322: The First Artificial Cloning Vector

pBR322 was the first artificial cloning vector created in the laboratory by Francisco Bolivar and Raymond L. Rodriguez. Compared to its pUC counterparts, pBR322 is larger in size, resulting in a slower rate of replication. This E. coli cloning vector contains an origin of replication (ori), restriction sites, and antibiotic-resistant genes.

pBR322 is a widely used cloning vector in E. coli with significant applications in molecular cloning.

pBR322 Nomenclature

• p: Plasmid
• BR: Bolivar and Rodriguez
• 322: Numerical designation

Constructed in 1977 in the laboratory of Herbert Boyer at the University of California, San Francisco, it was the first synthetic plasmid designed for use as a cloning vector. As one of the most studied plasmids, it is 4,362 base pairs long, fully sequenced, and has a molecular weight of 2.83 x 10⁶ Daltons.

Structure of pBR322

• Origin of replication
• Restriction enzyme sites
• Selectable marker sites

Origin of Replication

The origin of replication in pBR322 is known as pMB1, which maintains a copy number of 15–20.

Screening of Recombinants

When the restriction enzyme BamHI is used to cut the plasmid, it targets a position within the tetracycline resistance region. Consequently, the recombinant molecule containing the newly inserted DNA becomes sensitive to tetracycline but remains resistant to ampicillin.

To identify these cells:

1. Plate recombinant cells on a medium containing ampicillin.
2. Incubate to allow transformed colonies to appear.
3. Use the replica plate technique to confirm transformation.
4. Transfer colonies to a medium containing tetracycline.
5. Colonies that fail to grow on the tetracycline medium are confirmed as recombinants, as they have lost their tetracycline resistance.

Advantages of pBR322

• Manageable size: Ideal for standard cloning procedures.
• Selection markers: Two antibiotic resistance genes simplify the screening process.
• Versatility: Multiple restriction enzyme sites ensure compatibility.
• Copy number: Favorable for genetic engineering applications.

Disadvantages

The primary drawback is that the detection of recombinant cells containing the inserted DNA molecule can be time-consuming.