How Some Bacteria Incorporate Glutamine Without a Dedicated Glutaminyl-tRNA Synthetase

Aminoacyl-tRNA synthetases (aaRSs) are essential enzymes that attach specific amino acids to their corresponding tRNAs, ensuring accurate translation of the genetic code. Most organisms have 20 different aaRSs, one for each amino acid. However, some bacteria lack a dedicated glutaminyl-tRNA synthetase (GlnRS) for charging tRNA^Gln with glutamine. This raises the question: how do these bacteria incorporate glutamine into proteins?

The Indirect Pathway for Glutamine Incorporation

• In bacteria lacking GlnRS, the glutamyl-tRNA synthetase (GluRS) charges tRNA^Gln with glutamate instead of glutamine.

• This mischarged tRNA^Gln (carrying glutamate) is then modified by a second enzyme called glutamyl-tRNA^Gln amidotransferase, which converts the attached glutamate into glutamine while still bound to the tRNA.

• This two-step process is known as the indirect pathway of glutaminyl-tRNA formation.

• The result is a correctly charged tRNA^Gln with glutamine, ready for incorporation into the growing polypeptide chain during translation.

Why Other Options Are Incorrect

• Option 1: Glutamine is indeed incorporated during translation, not post-translationally modified from glutamate residues.

• Option 2: GluRS does not directly charge tRNA^Gln with glutamine; it charges it with glutamate.

• Option 4: The synthetase does not charge tRNA^Glu with glutamine; charging specificity is strict.

Biological Significance

• This indirect charging mechanism is common in many bacteria and archaea and is an example of evolutionary adaptation to enzyme availability.

• It ensures that glutamine is incorporated accurately despite the absence of a dedicated GlnRS.

• The amidotransferase enzyme plays a crucial proofreading role, preventing mistranslation.

Summary Table

Keywords for SEO Optimization

• Glutaminyl-tRNA synthetase absence

• Indirect glutamine incorporation pathway

• Glutamyl-tRNA synthetase and tRNA^Gln

• Glutamyl-tRNA amidotransferase function

• Bacterial translation mechanisms

• Aminoacyl-tRNA synthetase specificity

• tRNA charging and editing

• Protein synthesis in bacteria

• Evolution of aminoacyl-tRNA synthetases

• Translation fidelity in prokaryotes

Conclusion

In bacteria that lack a dedicated glutaminyl-tRNA synthetase, glutamine incorporation into proteins is achieved through an indirect pathway. The glutamyl-tRNA synthetase charges tRNA^Gln with glutamate, and a specialized amidotransferase enzyme converts the attached glutamate to glutamine. This elegant mechanism ensures accurate protein synthesis despite the absence of GlnRS.

Correct answer: (3) In these bacteria, the aminoacyl tRNAsynthetase specific for tRNAGlu also charges tRNAGln with glutamate. A second enzyme then converts the glutamate of the charged tRNAGln to glutamine.