The following statements are made with reference to the fact that tRNAs are known to possess T in their sequence. A. RNA polymerase III utilizes TTP as one of the substrates. B. Like any other RNA polymerase, RNA polymerase III also utilizes rUTP but when the RNA pol III reaches a looped structure it binds to S-adenosylmethionine to methylate C-5 position of the incorporated U to result in a thymine in a co-transcriptional manner. C. During transcription of tRNA genes at the designated positions, DNA polymerase replaces RNA polymerase III to incorporate T in the tRNA transcript D. A specific methyltransferase utilizes a methyl group donor to post- transcriptionally modify the specific U residues into T residues. E. Uracil to thymine conversion occurs in a large number of tRNAs in the TΨC loop. The option with all the correct statements is
D and E only
Let's analyze the statements provided regarding the presence of Thymine (T) in tRNA sequences. While DNA typically contains Adenine (A), Guanine (G), Cytosine (C), and Thymine (T), RNA typically contains A, G, C, and Uracil (U). However, tRNAs are known to contain many modified bases, including Ribothymidine, which is a methylated form of Uracil and is functionally equivalent to Thymine in terms of base pairing within RNA structure.
tRNA Statements Analysis
We will evaluate each statement to determine its accuracy in explaining how Thymine (Ribothymidine) appears in tRNA.
-
A. RNA polymerase III utilizes TTP as one of the substrates.
This statement is incorrect. RNA polymerases, including RNA polymerase III which transcribes tRNA genes, utilize ribonucleoside triphosphates (rNTPs) as substrates: ATP, UTP, CTP, and GTP. Deoxynucleoside triphosphates (dNTPs), like TTP, are substrates for DNA polymerases during DNA replication.
-
B. Like any other RNA polymerase, RNA polymerase III also utilizes rUTP but when the RNA pol III reaches a looped structure it binds to S-adenosylmethionine to methylate C-5 position of the incorporated U to result in a thymine in a co-transcriptional manner.
This statement describes a highly unusual and incorrect mechanism. While RNA polymerase III does use rUTP, the methylation of Uracil to form Ribothymidine is typically a post-transcriptional modification process carried out by specific enzymes, not by the RNA polymerase itself during transcription. The proposed co-transcriptional methylation mechanism linked to looped structures and direct binding of S-adenosylmethionine by the polymerase is not accurate.
-
C. During transcription of tRNA genes at the designated positions, DNA polymerase replaces RNA polymerase III to incorporate T in the tRNA transcript.
This statement is fundamentally incorrect. DNA polymerase is the enzyme responsible for synthesizing DNA during replication. RNA polymerase is responsible for synthesizing RNA during transcription. DNA polymerase does not participate in the transcription of tRNA genes, nor does it replace RNA polymerase III for incorporating bases into an RNA molecule.
-
D. A specific methyltransferase utilizes a methyl group donor to post- transcriptionally modify the specific U residues into T residues.
This statement is correct. The presence of Ribothymidine (often represented as T) in tRNA is a result of post-transcriptional modification. Specific enzymes called methyltransferases recognize certain Uracil residues within the transcribed tRNA molecule and add a methyl group (typically from a donor like S-adenosylmethionine, although the statement only mentions a donor generally) to the C-5 position of the uracil ring, converting it into Ribothymidine.
-
E. Uracil to thymine conversion occurs in a large number of tRNAs in the TΨC loop.
This statement is correct. The TΨC loop is one of the characteristic loops in the secondary structure of most tRNAs. The 'T' in TΨC refers specifically to Ribothymidine. The conversion of Uracil to Ribothymidine by methyltransferases frequently occurs in this specific loop region, which is important for the proper structure and function of the tRNA, including its interaction with ribosomes.
Correct Statements Conclusion
Based on the analysis, statements D and E accurately describe the mechanism for the presence of Ribothymidine (T) in tRNA. Statement D explains the enzymatic process of post-transcriptional methylation of Uracil, and statement E identifies a common location (TΨC loop) where this modification occurs in many tRNAs.
Important Questions from RNA synthesis and processing
Which one of the following statements related to transcription and processing of mRNA is INCORRECT?
Following statements were made with respect to transcription in eukaryotes:
A. RNA polymerase III synthesises mRNAs in the nucleoplasm
B. The target promoter for RNA polymerase III 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?
Following statements were made about the post-transcriptional processing of RNA in eukaryotes.
A. Soon after transcription initiation, RNA polymerase II pauses ~30 nucleotides downstream from the site of initiation until the Cap structure is added to the 5' end of the nascent pre- mRNA.
B. The 5' splice sites are functionally divergent whereas the 3' sites are functionally equivalent.
C. In addition to helping in recognition of the splice sites, the exon definition also functions as a splicing regulator by allowing pairing and linking of adjacent 5' and 3' splice sites.
D. The intron definition mechanism applies only to the larger introns (above 500 nucleotides length) and assists in achieving alternate splicing.
E. The splicing reactions carried out in vitro have revealed that the first and second transesterification reactions are reversible.
Which one of the following combination of statements is correct?
Which one of the following statements is NOT a correct feature of Escherichia coli RNA polymerase?
The following statements are related to transcription in bacteria/eukaryotes.
A. During concurrent promoter sequence recognition and melting, melting commences with base flipping where two bases are flipped out into pockets of the primary sigma factor
B. Binding of α-amanitin to RNA polymerase Il permits entry of nucleotides into RNA pol II active site and synthesis of RNA, but prevents translocation
C. RNA polymerase I can use upstream promoters with 3 consensus sequences, as well as internal promoters having a bipartite structure
D. FACT is associated with RNA polymerase during transcriptional elongation and helps displace histone octomers during transcription
Which of the following combinations of statements represents all correct statements?
