Question

Differentiate between RNA polymerase I, RNA polymerase II and RNA polymerase III.

Answer 1

Hint: The transcriptional mechanism in the nucleus of a eukaryotic cell is much more complex than that in the prokaryotic cell. Eukaryotes have three distinct RNA polymerases, designated as RNA polymerase I, II, and III, which are individual complexes but have certain subunits in common.

Complete Answer:
Each polymerase enzyme has a specific function and is employed for a specific promoter sequence.
The differences among the three RNA polymerases are as follows:

Sl.No	RNA polymerase I	RNA polymerase II	RNA polymerase III
1.	RNA polymerase I also known as Pol I is responsible for the transcription of an RNA molecule that is precursor for the ribosomal RNAs – 18S, 5.8S, and 28S rRNAs.	The principal function of RNA polymerase II also known as Pol II is transcription of mRNA, mi RNA, snRNA (small nuclear RNA), and snoRNA (small nucleolar RNA).	RNA polymerase III also known as Pol III transcribes tRNAs, the 5S rRNA, and some other small specialized RNAs.
2.	It resides in the nucleolus.RNA polymerase I consists of 14 subunits.	It resides in nucleoplasm. RNA polymerase II consists of 14 subunits.	It resides in nucleoplasm.RNA polymerase III is the largest eukaryotic RNA polymerase consisting of 17 subunits.
3.	Pol I promoters sequences differ significantly from one species to other species.	This enzyme can recognize a number of promoters that differ greatly in sequence. Several Pol II promoters have a few sequences in common, including a TATA box that is a eukaryotic consensus sequence TATAAA and an Initiator sequence.	The promoters identified by Pol III are well characterized. Some of the sequences required for the regulated initiation of transcription by Pol III are situated within the gene itself, whereas others are located upstream of the RNA start site.

Note: In prokaryotes, only one RNA polymerase transcribes for all types of RNAs, whereas, in eukaryotes three nuclear enzymes RNA polymerase I, II and III are utilized to synthesize different classes of RNA.