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PBAD promoter

PBAD promoter is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand PBAD promoter rather than just read about it. In short: PBAD (systematically araBp) is a promoter found in bacteria and especially as part of plasmids used in laboratory studies. The promoter is a part of the arabinose operon whose name derives from the genes it regulates transcription of: araB, araA, and araD.

PBAD promoter — main illustration
PBAD promoter — illustration

Key takeaways

  • PBAD promoter belongs to biology; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect PBAD promoter to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of PBAD promoter from memory before moving on to harder problems.

Reference excerpt

PBAD (systematically araBp) is a promoter found in bacteria and especially as part of plasmids used in laboratory studies. The promoter is a part of the arabinose operon whose name derives from the genes it regulates transcription of: araB, araA, and araD. In E. coli, the PBAD promoter is adjacent to the PC promoter (systematically araCp), which transcribes the araC gene in the opposite direction. araC encodes the AraC protein, which regulates activity of both the PBAD and PC promoters. The cyclic AMP receptor protein CAP binds between the PBAD and PC promoters, stimulating transcription of both when bound by cAMP.

Regulation of PBAD Transcription initiation at the PBAD promoter occurs in the presence of high L-arabinose and low glucose concentrations. Upon arabinose binding to AraC, the N-terminal arm of AraC is released from its DNA binding domain via a “light switch” mechanism. This allows AraC to dimerize and bind the I1 and I2 operators. The AraC-arabinose dimer at this site contributes to activation of the PBAD promoter. Additionally, CAP binds to two CAP binding sites upstream of the I1 and I2 operators and helps activate the PBAD promoter. In the presence of both high arabinose and high glucose concentrations however, low cAMP levels prevent CAP from activating the PBAD promoter. It is hypothesized that PBAD promoter activation by CAP and AraC is mediated through contacts between the C-terminal domain of the α-subunit of RNA polymerase and the CAP and AraC proteins.

Without arabinose, and regardless of glucose concentration, the PBAD and PC promoters are repressed by AraC. The N-terminal arm of AraC interacts with its DNA binding domain, allowing two AraC proteins to bind to the O2 and I1 operator sites. The O2 operator is situated within the araC gene. An AraC dimer also binds to the O1 operator and represses the PC promoter via a negative autoregulatory feedback loop. The two bound AraC proteins dimerize and cause looping of the DNA. The looping prevents binding of CAP and RNA Polymerase, which normally activate the transcription of both PBAD and PC.

The spacing between the O2 and I1 operator sites is critical. Adding or removing 5 base pairs between the O2 and I1 operator sites abrogates AraC mediated repression of the PBAD promoter. The spacing requirement arises from the double helix nature of DNA, in which a complete turn of the helix is about 10.5 nucleotides. Therefore, adding or removing 5 base pairs between the O2 and I1 operator sites rotates the helix roughly 180 degrees. This reverses the direction that the O2 operator faces when the DNA is looped and prevents dimerization of the O2 bound AraC with the bound I1 araC.

The PBAD promoter on expression plasmids

The PBAD promoter allows for tight regulation and control of a target gene in vivo. As explained above, PBAD is regulated by the addition and absence of arabinose. As tested, the promoter can be further repressed with reduced levels of cAMP through the addition of glucose. Plasmid vectors have been constructed and tested with a selectable marker (CmR in this case), origin of replication, araC and operons, multiple cloning site and PBAD promoter. Studies show that vectors are highly expressed and can be used, in combination with chromosomal null alleles, to study loss of function of essential genes.

References

Illustrations

PBAD promoter: Figure 1. Expression of araB, araA and araC in the presence of arabinose. In the presence of L-arabinose, arabinose binds to the arabinose binding pocket sites of AraC, causing AraC to dimerize at the I1 and I2 operators. This allows access for CAP to bind to the CAP-binding sites, which in turn helps recruit RNA Polymerase to both PBAD and PC promoters and activates transcription.
Figure 1. Expression of araB, araA and araC in the presence of arabinose. In the presence of L-arabinose, arabinose binds to the arabinose binding pocket sites of AraC, causing AraC to dimerize at the I1 and I2 operators. This allows access for CAP to bind to the CAP-binding sites, which in turn helps recruit RNA Polymerase to both PBAD and PC promoters and activates transcription.
PBAD promoter: Figure 2. Expression of araB, araA and araC does not occur when arabinose is not present. In the absence of arabinose, AraC dimerizes while bound to the O2 and I1 operator sites, looping the DNA. The looping prevents binding of CAP and RNA Polymerase, which normally activate the transcription of both PBAD and PC.
Figure 2. Expression of araB, araA and araC does not occur when arabinose is not present. In the absence of arabinose, AraC dimerizes while bound to the O2 and I1 operator sites, looping the DNA. The looping prevents binding of CAP and RNA Polymerase, which normally activate the transcription of both PBAD and PC.
PBAD promoter: Figure 3. A representation of the PBAD 33 promoter on a plasmid with common cis-acting regions. Abbreviations are defined as the phagemid origin (f1 origin), chloramphenicol resistance (CmR), plasmid origin (p15A ori), araC gene (araC), araC operator sites (araC O2 and O1), CAP-binding site (CAP BS), araC inducer sites (I1/I2), PBAD promoter (pBAD) and the multiple cloning site (MCS).
Figure 3. A representation of the PBAD 33 promoter on a plasmid with common cis-acting regions. Abbreviations are defined as the phagemid origin (f1 origin), chloramphenicol resistance (CmR), plasmid origin (p15A ori), araC gene (araC), araC operator sites (araC O2 and O1), CAP-binding site (CAP BS), araC inducer sites (I1/I2), PBAD promoter (pBAD) and the multiple cloning site (MCS).

Worked examples

Example 1 — a first encounter with PBAD promoter

Start with the simplest possible case. Write down what PBAD promoter claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to PBAD promoter before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about PBAD promoter ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of PBAD promoter

In research
PBAD promoter appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses PBAD promoter in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
PBAD promoter is common in secondary-school and first-year university syllabi. It links to neighbouring topics Gene expression, so understanding it makes those chapters shorter.
In everyday life
Look for PBAD promoter outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study PBAD promoter in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what PBAD promoter means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain PBAD promoter out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is PBAD promoter in simple terms?

PBAD (systematically araBp) is a promoter found in bacteria and especially as part of plasmids used in laboratory studies. The promoter is a part of the arabinose operon whose name derives from the genes it regulates transcription of: araB, araA, and araD.

Why does PBAD promoter matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study PBAD promoter?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on PBAD promoter.

Tags

  • Gene expression

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