23. Following statements were made with respect to transcription in eukaryotes:
A. RNA polymerase III synthesizes mRNAs in the nucleoplasm
B. The target promoter for RNA polymerase Ill is usually represented by a bipartite sequence downstream of the transcription start site.
C. The assembly factors TFIIIA and TFIIIC assist the binding of the positioning factor TFIIIB at the precise location.
D. TFIIIB is the last factor that joins the initiation complex.
E. Phosphorylated Ser residues in the C-terminal domain (CTD) of RNA polymerase II serve as binding sites for mRNA processing enzymes.
Which one of the following options represents the correct combination of the statements?
(1) A, B and C         (2) B, C and E
(3) B, D and E         (4) A, C and E

Correct Combination of Statements About Eukaryotic Transcription by RNA Polymerase III

Question Recap:

Statements about eukaryotic transcription:

A. RNA polymerase III synthesizes mRNAs in the nucleoplasm
B. The target promoter for RNA polymerase III is usually a bipartite sequence downstream of the transcription start site
C. Assembly factors TFIIIA and TFIIIC assist the binding of the positioning factor TFIIIB at the precise location
D. TFIIIB is the last factor that joins the initiation complex
E. Phosphorylated Ser residues in the C-terminal domain (CTD) of RNA polymerase II serve as binding sites for mRNA processing enzymes

Choose the correct combination:
(1) A, B and C
(2) B, C and E
(3) B, D and E
(4) A, C and E

Correct Answer:

(2) B, C and E

Detailed Explanation:

A. RNA polymerase III synthesizes mRNAs in the nucleoplasm
This is false. RNA polymerase III transcribes small non-coding RNAs such as tRNAs, 5S rRNA, U6 snRNA, and other small RNAs, not mRNAs. mRNAs are synthesized by RNA polymerase II.

B. The target promoter for RNA polymerase III is usually a bipartite sequence downstream of the transcription start site
This is true. Pol III promoters are often intragenic (within the transcribed region) and bipartite, such as the A and B boxes in tRNA genes, located downstream of the transcription start site.

C. Assembly factors TFIIIA and TFIIIC assist the binding of the positioning factor TFIIIB at the precise location
This is true. For Pol III transcription, TFIIIA and TFIIIC bind to internal promoter elements and recruit TFIIIB, which positions RNA polymerase III at the correct transcription start site.

D. TFIIIB is the last factor that joins the initiation complex
This is false. TFIIIB assembles before RNA polymerase III and is responsible for recruiting the polymerase to the promoter. It is not the last factor; RNA polymerase III joins after TFIIIB.

E. Phosphorylated Ser residues in the CTD of RNA polymerase II serve as binding sites for mRNA processing enzymes
This is true. The CTD of RNA polymerase II is phosphorylated on serine residues during transcription initiation and elongation, creating docking sites for capping enzymes, splicing factors, and polyadenylation machinery.

Summary Table

Conclusion

The correct combination of true statements is B, C, and E (Option 2). RNA polymerase III transcribes small RNAs from bipartite internal promoters, with TFIIIA and TFIIIC recruiting TFIIIB. Meanwhile, RNA polymerase II’s CTD phosphorylation coordinates mRNA processing.