Question

Which of the following can add the terminal 5'CCA3' at the end of mature tRNA?

(1). tRNA nucleotidyltransferase
(2). RNase D
(3). RNase P
(4). RNase E/F

• A. tRNA nucleotidyltransferase
• B. RNase D
• C. RNase P
• D. RNase E/F

Hint:

The CCA-adding enzyme (tRNA nucleotidyltransferase) is unique because it is a template-independent RNA polymerase.
It adds the C-C-A sequence to the 3' end of the tRNA without requiring a DNA template.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
During the biogenesis of transfer RNA (tRNA), precursor molecules must undergo multiple post-transcriptional processing steps to become functional adapter molecules.
A universal requirement for all functional tRNAs is the presence of a conserved 5'-CCA-3' sequence at their 3'-terminus, which is required for amino acid charging by aminoacyl-tRNA synthetases.

Step 2: Detailed Explanation:
Let us look at the role of each enzyme in tRNA processing:
1. tRNA nucleotidyltransferase (1): This enzyme, also known as the CCA-adding enzyme, is responsible for the template-independent addition of the 5'-CCA-3' sequence to the 3' end of immature tRNA molecules.
It uses CTP and ATP as substrates to add two cytosines and one adenine residue to the processed 3' end of the tRNA without requiring a DNA template. This enzyme is essential in eukaryotes and prokaryotes whose tRNA genes do not encode the CCA sequence.
2. RNase P (3): This is an endonuclease ribozyme that cleaves the 5' leader sequence of precursor tRNAs.
3. RNase D (2): This is a 3'-to-5' exonuclease involved in trimming the 3' end of precursor tRNAs, preparing them for CCA addition.
4. RNase E/F (4): These are endonucleases involved in the initial processing and separation of polycistronic tRNA transcripts in bacteria.
Therefore, only tRNA nucleotidyltransferase can synthesize and add the terminal 5'-CCA-3' sequence.

Step 3: Final Answer:
The correct enzyme is tRNA nucleotidyltransferase, which corresponds to option (A).