Question:

For expression of eukaryotic gene in a prokaryotic system, which sequence has to be inserted from its first codon? (The question phrasing "inserted from its first codon" is a bit unusual. It likely means "inserted *before* its first codon" or what sequence is needed upstream for prokaryotic expression machinery to recognize it.)

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  • Shine-Dalgarno Sequence (Prokaryotes) Ribosome binding site on mRNA, upstream of start codon. Pairs with 16S rRNA.
  • Kozak Sequence (Eukaryotes) Consensus sequence around the start codon (AUG) that enhances translation initiation efficiency. Ribosome binds 5' cap and scans.
  • For heterologous gene expression (e.g., eukaryotic gene in prokaryote), appropriate regulatory signals of the host system must be provided.
Updated On: May 22, 2025
  • Kozak sequence
  • Shine Dalgarno sequence
  • Enhancers
  • Silencers
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The Correct Option is B

Solution and Explanation

For the expression of a gene in a host system, specific regulatory sequences are required for transcription and translation initiation.
  • Prokaryotic Translation Initiation: In prokaryotes (like E. coli), the ribosome (specifically the 16S rRNA in the 30S small subunit) binds to a specific sequence on the mRNA called the Shine-Dalgarno (SD) sequence. This sequence is typically located a few nucleotides upstream (5') of the start codon (usually AUG). The SD sequence helps to correctly position the ribosome on the mRNA for translation initiation.
  • Eukaryotic Translation Initiation: In eukaryotes, the small ribosomal subunit (40S) typically binds at the 5' cap of the mRNA and then scans along the mRNA until it encounters the start codon (AUG). The efficiency of initiation can be influenced by the sequence context around the start codon, known as the Kozak sequence (e.g., (gcc)gccRccAUGG, where R is a purine).
The question asks about expressing a eukaryotic gene in a prokaryotic system. Therefore, for the prokaryotic ribosome to recognize and translate the eukaryotic mRNA (produced from the eukaryotic gene), a prokaryotic ribosome binding site, i.e., a Shine-Dalgarno sequence, must be present upstream of the start codon of the eukaryotic coding sequence. This SD sequence needs to be engineered into the expression vector if the eukaryotic gene's natural context doesn't provide one that works in prokaryotes. Let's analyze the options: (a) Kozak sequence: Important for translation initiation in eukaryotes. (b) Shine Dalgarno sequence: Required for ribosome binding and translation initiation in prokaryotes. This is the correct answer. (c) Enhancers: DNA sequences that increase the rate of transcription of a gene in eukaryotes. They are cis-acting regulatory elements for transcription, not directly translation initiation. (d) Silencers: DNA sequences that decrease or repress the transcription of a gene in eukaryotes. Therefore, for expression of a eukaryotic gene in a prokaryotic system, a Shine-Dalgarno sequence needs to be appropriately positioned upstream of the start codon. \[ \boxed{\text{Shine Dalgarno sequence}} \]
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