Consider the even-skipped regulatory sequences in Figure 18.9.
Consider the binding sites for gap proteins and Bicoid in the stripe 2 enhancer module. What sites are occupied in parasegments 2, 3, and 4, and how does this result in expression or no expression?

Verified step by step guidance

1. Step 1: Understand the biological context—'even-skipped' (eve) is a segmentation gene in Drosophila, and its stripe 2 enhancer module controls expression in specific parasegments by integrating inputs from gap proteins and Bicoid, which are transcription factors that bind to regulatory DNA sequences.
2. Step 2: Identify the binding sites for gap proteins (such as Hunchback, Kruppel, Giant) and Bicoid within the stripe 2 enhancer module, noting which proteins act as activators and which act as repressors for eve expression.
3. Step 3: For each parasegment (2, 3, and 4), determine the presence or absence of these proteins based on their expression gradients and domains, then infer which binding sites are occupied by activators or repressors in that parasegment.
4. Step 4: Analyze how the combination of occupied activator and repressor sites in each parasegment influences the transcriptional activity of the stripe 2 enhancer—activator binding promotes expression, while repressor binding inhibits it.
5. Step 5: Conclude for each parasegment whether eve stripe 2 is expressed or not, based on the net effect of bound proteins at the enhancer, explaining how the spatial pattern of protein binding leads to the observed stripe pattern.

Key Concepts

Even-skipped (eve) Stripe 2 Enhancer Module

The even-skipped gene contains specific regulatory DNA sequences called enhancers that control its expression in distinct stripes during embryonic development. The stripe 2 enhancer integrates inputs from various transcription factors to activate eve expression precisely in parasegment 2. Understanding this module is key to linking protein binding patterns to spatial gene expression.

Gap Proteins and Bicoid as Transcriptional Regulators

Gap proteins (such as Kruppel and Giant) and Bicoid are maternal and zygotic transcription factors that bind to specific sites on enhancers. Bicoid generally acts as an activator, while gap proteins can act as activators or repressors depending on context. Their binding patterns in different parasegments determine whether the target gene is expressed or repressed.

Spatial Regulation of Gene Expression in Parasegments

Parasegments are embryonic units with distinct gene expression profiles. The combination of transcription factors bound to the stripe 2 enhancer varies across parasegments 2, 3, and 4, leading to differential gene expression. This spatial regulation ensures that even-skipped is expressed only where activators dominate and repressors are absent or minimal.