Q.42. Absence of detectable protein expression upon blunt-ended mutation-free cloning of an E.coli gene
with its own promoter in E. coli cells can be due to
(A)Cloning occurred in reverse orientation
(B) Cloning occurred out of frame
(C) Codon bias
(D) Rapid degradation of expressed protein
Absence of detectable protein expression in blunt-ended, mutation-free cloning of an E. coli gene with its native promoter often stems from orientation issues. This SEO-optimized article breaks down the correct answer and all options for molecular biology students preparing for exams like GATE Life Sciences.
Correct Answer
Cloning occurred in reverse orientation (Option A) prevents detectable protein expression. In blunt-end ligation, the insert lacks directional restriction sites, yielding a 50:50 chance of forward or reverse insertion. Reverse orientation positions the gene’s coding strand as the template, blocking transcription from the upstream promoter and yielding antisense RNA instead of mRNA. E. coli promoters drive transcription only in the correct orientation, so reverse clones produce no functional protein despite intact sequences.
Option Explanations
Blunt-ended cloning ensures no mutations or frameshifts from sticky ends, ruling out certain errors, but other factors persist.
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(A) Cloning occurred in reverse orientation: Primary cause here. Native promoter fails without correct 5′-3′ gene alignment; vectors like pGRASS screen this via GFP loss in forward clones.
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(B) Cloning occurred out of frame: Unlikely in blunt-end ligation of PCR-amplified native gene, as it starts at ATG without vector fusion. Frameshifts need indels, excluded by “mutation-free”.
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(C) Codon bias: Irrelevant for native E. coli gene, optimized for its own tRNA pool. Bias affects heterologous genes, not self-expression.
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(D) Rapid degradation of expressed protein: Possible generally via cytoplasmic proteases, but undetected expression means no protein forms first. Periplasmic secretion or stabilizers mitigate this, not applicable without transcription.
Key Implications for Cloning
Reverse orientation plagues blunt-end strategies, as ligation ignores directionality. Solutions include directional cloning (e.g., restriction enzymes) or screening vectors detecting orientation via reporters. For E. coli native genes, verify by colony PCR or sequencing post-ligation. This error explains ~50% failed clones in non-directional setups.
