Which Codon Codes for Selenocysteine in Escherichia coli?

Selenocysteine (Sec), known as the 21st amino acid, is uniquely incorporated into proteins by a specialized mechanism that redefines the genetic code. In Escherichia coli, the codon responsible for encoding selenocysteine is distinct from the standard amino acid codons.

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The Role of UGA Codon in Selenocysteine Incorporation

• The UGA codon, normally a stop codon signaling termination of translation, is recoded to specify selenocysteine insertion in E. coli and many other organisms.

• This recoding requires a specific mRNA secondary structure called the selenocysteine insertion sequence (SECIS) element, located downstream of the UGA codon.

• The specialized elongation factor SelB recognizes the SECIS element and delivers the selenocysteyl-tRNA^Sec to the ribosome at the UGA codon, allowing incorporation of Sec instead of terminating translation.

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Why Other Codons Are Not Used

• UAA and UAG are also stop codons but are not used for selenocysteine incorporation in E. coli.

• UCC codes for serine and is unrelated to selenocysteine insertion.

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Summary Table

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Keywords for SEO Optimization

• Selenocysteine codon in E. coli

• UGA codon recoding

• SECIS element and selenocysteine insertion

• SelB elongation factor function

• Selenoprotein biosynthesis in bacteria

• Genetic code expansion with selenocysteine

• Translation termination codons

• Stop codon reassignment

• Protein synthesis with selenocysteine

• Molecular biology of selenoproteins

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Conclusion

In Escherichia coli, the codon UGA is uniquely repurposed to encode selenocysteine during translation, facilitated by the SECIS element and SelB elongation factor. This recoding allows the incorporation of the 21st amino acid at specific sites within selenoproteins.

Correct answer: (1) UGA