Question

Consider the signaling pathway depicted below, which involves two different receptors. Assume that a protein is active only when it is activated but not inhibited by its upstream signaling molecules. Activation and inhibition are indicated by normal and blunt arrows, respectively. Under which of the following conditions is target-gene expression induced?

A. Only in the presence of both signals (1 and 2)
B. In the presence of either or both signals (1 or 2)
C. In the presence of either signal but not both of them
D. Only in the absence of both signals
E. Only in the presence of signal 2 but not signal 1

Answer 1

Target-gene expression is induced 'In the presence of either signal but not both of them', as the provided scenario suggests that the combined presence of both signals might result in inhibition, contrary to when they are present individually.

Step-by-step explanation:

To understand how target-gene expression is induced in the context of a signaling pathway involving two different receptors, we need to consider the mechanisms of signal transduction and the conditions specified in the question.

Within such pathways, the activation of signaling molecules proceeds through a chain of events, sometimes described as a signaling cascade.

This cascade involves upstream events, where second messengers, enzymes, and activated proteins cause further activations downstream. The culmination of these events can result in changes to cell behavior, including metabolism and gene expression.

Signal integration is particularly relevant here, as it involves the merging of signals from multiple pathways, allowing the cell to respond adequately only after all requirements are fulfilled.

Given that a protein in this pathway is active only when activated and not inhibited, target-gene expression would be induced under the specific conditions where the upstream signals allow for it.

Considering the options provided and understanding that the signal must be activated and not inhibited, the correct answer in this context would be 'C. In the presence of either signal but not both of them'.

This indicates that the presence of both signals might lead to an inhibition that would prevent the activation required for target-gene expression.