The three promoters are functionally interconnected through their shared regulatory elements, leading to the coordinated but hierarchical expression of AAV’s replication (Rep) and capsid (Cap) genes.
| Promoter | Cis-Acting Elements (Key Sites) | Key Trans-Acting Factors | Regulation (During Lytic Infection) |
| P5 | Rep Binding Element (RBE), YY1 site, ATF, MLTF | Rep78/68, Adenovirus E1A, YY1, MLTF | Primarily Repressed by Rep78/68 binding to the P5 RBE. Activated by Adenovirus E1A and Rep binding to the RBE in the ITR. |
| P19 | Sp1-50 site, CArG-like element, TATA box | Rep78/68, Sp1, Adenovirus E1A | Transactivated by Rep78/68. Requires the upstream P5 RBE or ITR RBE to facilitate Rep/Sp1 interaction. |
| P40 | Sp1-50 site, GGT-70 site, TATA box, ATF-80, AP1-40 | Rep78/68, Sp1, Adenovirus E1A | Strongly Transactivated by Rep78/68. The Rep protein, bound upstream, interacts with the P40 Sp1-50 site and the P19 CArG-140 element (acting as a remote enhancer) to form a DNA loop, which is critical for maximal activity. |
1. Rep Binding Element (RBE) as the Master Switch
The core of AAV’s transcriptional regulation lies in the Rep Binding Element (RBE), a 22-bp sequence.
- Location: The RBE is present within the $\text{P5}$ promoter and also in the Inverted Terminal Repeats (ITRs).
- Repressor & Activator: The large Rep proteins (Rep78/68) bind to the RBE and act as both a repressor and an activator:
- Binding to the P5 RBE represses the P5 promoter, limiting the production of Rep proteins.
- Binding to the P5 RBE or the ITR RBE transactivates the P19 and P40 promoters, creating the necessary concentration gradient for the viral life cycle (P40 >> P19 > P5).
2. Coordination of P19 and P40 Activation
The strong activation of P19 and P40 by Rep78/68 is a long-range phenomenon, requiring a physical interaction between distantly bound Rep protein and the proximal transcription start sites:
- P19 & P40 Activation: Rep (bound at the upstream RBE) interacts with the Sp1-50 site located within the P19 and P40 promoters, likely by forming a DNA loop. This mechanism brings the RBE-Rep complex close to the P19 and P40 transcription initiation machinery, facilitating a very high level of P40 expression.
- P40 Specificity: The activation of P40 requires an interaction between Rep, the P40 Sp1-50 element, and a CArG-like element within the P19 promoter region, which functions as an important cis-element for P40 activation.
3. Role of Helper Virus Factors
During a lytic infection, helper viruses (like Adenovirus) provide key factors that overcome the host’s normal repression mechanisms and enhance AAV transcription:
- Adenovirus E1A is a crucial factor that acts to transactivate the AAV promoters, particularly P5, initially overriding cellular repression.
- Cellular factors like YY1 (Yin-Yang 1) and MLTF (Major Late Transcription Factor) are also involved in regulating P5 activity, and their influence is modulated by both Rep and helper virus proteins.