Cis-Acting Elements and Trans-Acting Factors on AAV2 Promoters

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.