Q.29 The 5’ ends of the mature forms of the prokaryotic mRNAs and tRNAs are
(A) a triphosphate group in mRNAs and a monophosphate group in tRNAs
(B) triphosphate groups in both mRNAs and tRNAs
(C) monophosphate groups in both mRNAs and tRNAs
(D) a monophosphate group in mRNAs and a triphosphate group in tRNAs
5′ Ends of Prokaryotic mRNA and tRNA: Triphosphate in mRNA, Monophosphate in tRNA
Prokaryotic mRNAs retain a 5′ triphosphate group from transcription initiation, while mature tRNAs acquire a 5′ monophosphate through RNase P processing. This distinction arises from their distinct maturation pathways in bacteria like E. coli. The correct answer to the multiple-choice question is option (A).
Correct Answer
The 5’ ends of mature prokaryotic mRNAs carry a triphosphate group (pppN-), directly from RNA polymerase initiation without capping. In contrast, mature tRNAs feature a 5′ monophosphate (pN-) generated by ribonuclease P (RNase P) cleavage of precursor leader sequences. This makes option (A) correct: a triphosphate group in mRNAs and a monophosphate group in tRNAs.
Option Analysis
(A) Triphosphate in mRNAs, Monophosphate in tRNAs
Prokaryotic mRNA transcription starts with a purine nucleoside triphosphate, yielding a 5′ triphosphate end that remains unmodified in the mature form for rapid translation. tRNA precursors undergo 5′ leader removal by RNase P, producing the characteristic 5′ monophosphate essential for aminoacylation and stability.
(B) Triphosphate Groups in Both
Incorrect, as tRNAs lose the triphosphate during maturation; RNase P endonucleolytically cleaves to generate monophosphate, unlike unprocessed mRNA.
(C) Monophosphate Groups in Both
Wrong for mRNA, which lacks 5′ processing enzymes like those in eukaryotes; prokaryotic mRNAs retain the primary 5′ triphosphate for polycistronic translation initiation.
(D) Monophosphate in mRNAs, Triphosphate in tRNAs
Opposite of reality; mRNAs are unprocessed at the 5′ end, while tRNAs specifically gain monophosphate post-transcriptionally.
Biological Relevance
These 5′ end structures influence decay rates and ribosome binding in prokaryotes. Triphosphate mRNAs degrade faster via pyrophosphohydrolase enzymes, aiding short half-lives (~2-5 minutes). Monophosphate tRNAs ensure structural integrity for transfer RNA function in protein synthesis. For biotech students, understanding this aids in recombinant protein expression systems using bacterial hosts.
