2. Which of the following options regarding an mRNA formed by bacterial transcription is correct?
a. The transcription start site occurs a few bases before the start codon.
b. The first 3 bases of the mRNA have to be 5’AUG3’.
c. The transcription start site occurs a few bases after the start codon.
d. The last 3 bases of the mRNA always correspond to one of the three stop codons.
The correct answer is a. The transcription start site occurs a few bases before the start codon. In bacterial transcription, RNA polymerase initiates at the +1 transcription start site (TSS) within the promoter, typically 5-10 nucleotides upstream of the AUG start codon, producing a 5′ untranslated region (5′ UTR) with Shine-Dalgarno sequence for ribosome binding.
Prokaryotic Transcription Basics
Bacterial mRNA synthesis begins at the TSS, positioned after the -10 and -35 promoter boxes, upstream of the coding sequence. This places the TSS before the start codon, allowing formation of ribosome binding sites in the leader sequence. Unlike eukaryotes, bacterial mRNA is polycistronic and translated directly, but always features this upstream TSS.
Option Analysis
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a. The transcription start site occurs a few bases before the start codon: Correct; primary TSS is upstream of the translational start, creating 5′ UTR for regulatory elements like Shine-Dalgarno.
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b. The first 3 bases of the mRNA have to be 5’AUG3’: Incorrect; mRNA starts with promoter-derived nucleotides (often purine), not AUG, which is downstream in the open reading frame.
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c. The transcription start site occurs a few bases after the start codon: Incorrect; TSS precedes the start codon; post-start sites would disrupt translation initiation.
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d. The last 3 bases of the mRNA always correspond to one of the three stop codons: Incorrect; transcription terminates at terminator sequences (rho-dependent or intrinsic), often downstream of stop codons, adding 3′ UTR.
Implications for CSIR NET Preparation
This question tests understanding of coupled transcription-translation in bacteria, where TSS positioning ensures efficient ribosome recruitment. Secondary TSS may occur internally, but primary is upstream. Aspirants should diagram promoter-TSS-start codon layout for exams.
