A yeast strain has accumulated a mutation that makes it grow slowly. Investigation reveals that ribosomal RNA levels have dropped drastically in this strain. Which RNA polymerase is likely to be mutated in this strain?

Correct option is A

RNA Polymerase I is responsible for synthesizing the 45S precursor rRNA, which is processed into 18S, 5.8S, and 28S rRNAs—the major structural components of ribosomes. A mutation in RNA Pol I leads to a drastic reduction in rRNA synthesis, impairing ribosome assembly, thereby slowing cell growth. Since the observed phenotype is a sharp drop in rRNA and reduced growth rate, RNA Pol I is the most likely enzyme affected.

Information Booster:

1. RNA Pol I synthesizes major rRNAs (18S, 5.8S, 28S) from a single 45S pre-rRNA.
2. rRNAs form the core of ribosomes, essential for protein synthesis.
3. RNA Pol I operates in the nucleolus.
4. rRNA production is directly proportional to growth rate and protein synthesis capacity.
5. RNA Pol I mutations impair ribosome biogenesis, leading to growth defects.
6. RNA Pol I transcription accounts for >50% of total cellular transcription activity in rapidly dividing cells.
7. Ribosome synthesis is a key rate-limiting step in cell proliferation.

Additional Information:

• (b) RNA Pol II transcribes mRNA, snRNA, and miRNA, not major rRNAs.
• (c) RNA Pol III synthesizes 5S rRNA, tRNAs, and other small RNAs, but 5S rRNA is a minor component.
• (d) RNA Pol IV is plant-specific, involved in siRNA synthesis; not present in yeast.

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