Eukaryotic pre-rRNA Transcription

In eukaryotes, the transcription of specific genes encoding ribosomal RNAs (rRNAs) is carried out by distinct RNA polymerases.

• RNA polymerase I is specialized for transcribing the genes for the major ribosomal RNAs: 18S, 5.8S, and 28S rRNAs. These are initially synthesized as a single large precursor molecule (pre-rRNA).
• RNA polymerase II transcribes protein-coding genes (mRNAs) and some small nuclear RNAs.
• RNA polymerase III transcribes transfer RNA (tRNA) genes, the 5S rRNA gene, and other small RNAs.
• RNA polymerase IV is primarily involved in small interfering RNA (siRNA) synthesis in plants.

The question specifically asks for the enzyme transcribing the precursor ribosomal RNAs for 28S, 18S, and 5.8S rRNAs. This function is exclusively performed by RNA polymerase I.

Identifying the Correct RNA Polymerase

Based on the known functions of eukaryotic RNA polymerases:

• Option A correctly identifies RNA polymerase I.
• Option B is incorrect as RNA polymerase II transcribes mRNA precursors.
• Option C is incorrect as RNA polymerase III transcribes tRNAs and 5S rRNA.
• Option D is incorrect as RNA polymerase IV has a different role in gene regulation.

Therefore, RNA polymerase I is the enzyme responsible for transcribing the eukaryotic precursor ribosomal RNAs (pre-rRNAs) encoding 28S, 18S, and 5.8S rRNAs.