Final answer:
The "bait" protein fused to the Gal4 DNA-binding domain does not activate reporter gene expression on its own because transcription activation requires both the DNA-binding domain and the activator domain.
Step-by-step explanation:
The reason the "bait" protein fused to the Gal4 DNA-binding domain cannot activate reporter gene expression on its own is because the full activation of transcription requires not just the binding domain (BD) but also the activator domain (AD). In the two-hybrid screening technique, the bait protein is fused to the BD, which enables it to bind to the promoter. However, this binding alone is insufficient to trigger transcription; the AD must also be present and functional to activate transcription. The prey protein is attached to the AD, and only if the prey protein successfully interacts (or "catches") with the bait can the two domains come together to complete the transcription factor and activate transcription of the reporter gene.
If the bait does not interact with the prey, meaning the AD is not brought into proximity with the BD, the transcription machinery remains inactive, and the reporter gene is not expressed. This is how scientists can discern whether two proteins interact based on the outcome of reporter gene expression.