Correct option is B
Explanation-
The transferrin receptor (TfR) helps cells uptake iron by binding transferrin-bound iron and internalizing it. Cells regulate TfR expression depending on iron availability, not at the transcriptional level, but at the mRNA stability level — a form of post-transcriptional regulation.
A. TfR mRNA increases 30-fold in absence of iron - This happens because iron regulatory proteins (IRPs) bind to iron response elements (IREs) in the 3′ UTR of the TfR mRNA. This prevents degradation of the mRNA → accumulation.
B. Mutations in the 3′UTR prevent this mRNA increase - This confirms that 3′UTR is the regulatory region. Mutations here impair IRE-IRP interaction, preventing stabilization.
C. High TfR mRNA despite α-amanitin treatment (a transcription inhibitor) - mRNA remains high in low iron even when transcription is blocked, showing regulation is post-transcriptional (mRNA is stabilized, not newly transcribed).
D. TfR mRNA decreases rapidly when iron is added - Iron causes IRPs to release the 3′UTR → exposes it to endonucleolytic degradation, leading to mRNA decay.
Final answer -
Transferrin receptor expression is regulated post-transcriptionally via elements in the 3′UTR, which modulate mRNA stability in response to iron — making Option b the most accurate choice.
Option b - Transferrin receptor is post-transcriptionally regulated, and the 3’UTR is the regulatory site.
