How do signal transduction pathways influence gene expression?

• A. External signals degrade transcription factors to silence genes.
• B. Signaling pathways only affect chromatin structure, not transcription factors.
• C. Signaling always increases transcription of all genes.
• D. External signals activate transcription factors via phosphorylation and other modifications, changing gene expression patterns.

Answer: (D)

Signal transduction shapes which genes are turned on or off by changing the activity of transcription factors in response to external cues. When a signal is detected, receptors trigger cascades of phosphorylation and other modifications that alter transcription factors—changing their ability to bind DNA, their stability, or their location (often moving into the nucleus). This can recruit coactivators to activate particular genes or recruit repressors or chromatin-modifying enzymes to silence others, producing a specific pattern of gene expression. A common example is kinases in pathways like MAPK or PKA modifying transcription factors so they drive target genes that respond to the signal. Because gene expression changes are context-dependent, signals do not simply increase transcription of all genes or universally degrade transcription factors; they selectively influence certain transcription factors to yield the appropriate expression pattern.