Practice question

Selective absence of protein in untreated samples and in magnesium transporter mutants, with induction only upon magnesium supplementation, indicates regulation beyond messenger RNA synthesis. If transcriptional control operated, mRNA levels would parallel protein induction. Splicing defects would generate size variants detectable on Northern blot, and increased mRNA decay would reduce total transcript abundance uniformly. Here transcript may be constitutively present but translationally silent due to 5' untranslated region riboswitch, upstream open reading frame, or magnesium-dependent translation factor. Magnesium influx relieves blockade, permitting ribosome loading and protein synthesis without changing mRNA quantity.