Perspectives and Insights into the Competition for Aminoacyl-tRNAs between the Translational Machinery and for tRNA Dependent Non-Ribosomal Peptide Bond Formation.

Angela Fung,Richard Fahlman,Roshani Payoe

doi:10.3390/life6010002

Angela Fung, Richard Fahlman + Show 1 more

Open Access

https://doi.org/10.3390/life6010002

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Abstract

Aminoacyl-tRNA protein transferases catalyze the transfer of amino acids from aminoacyl-tRNAs to polypeptide substrates. Different forms of these enzymes are found in the different kingdoms of life and have been identified to be central to a wide variety of cellular processes. L/F-transferase is the sole member of this class of enzyme found in Escherichia coli and catalyzes the transfer of leucine to the N-termini of proteins which result in the targeted degradation of the modified protein. Recent investigations on the tRNA specificity of L/F-transferase have revealed the unique recognition nucleotides for a preferred Leu-tRNALeu isoacceptor substrate. In addition to discussing this tRNA selectivity by L/F-transferase, we present and discuss a hypothesis and its implications regarding the apparent competition for this aminoacyl-tRNA between L/F-transferase and the translational machinery. Our discussion reveals a hypothetical involvement of the bacterial stringent response that occurs upon amino acid limitation as a potential cellular event that may reduce this competition and provide the opportunity for L/F-transferase to readily increase its access to the pool of aminoacylated tRNA substrates.

Highlights

The evolutionary sequence diversity of tRNAs is constrained by the selective pressures to maintain key nucleotides recognized by a variety of cellular factors, such as aminoacyl-tRNA synthetases, translation factors and a variety of other factors that utilize tRNAs for functions beyond that of protein synthesis
During leucine starvation (2–45 min) in RelA+ auxotroph strain CP78, the aminoacylation levels of tRNALeu isoacceptors have been measured and categorized: tRNALeu (CAA) and (UAA) isoacceptors are insensitive to amino acid starvation with aminoacylation levels of 10%–30%, tRNALeu (CAG) isoacceptor is categorized as intermediately sensitive to starvation with an aminoacylation level of 8%, and tRNALeu (GAG) and (UAG) isoacceptors are sensitive to starvation with an aminoacylation level of 2%–4% [86]
When considering the inhibition of EF-Tu and overall translation by (p)ppGpp, we hypothesize that the induction of the stringent response may provide L/F-transferase access to increased amounts of available aminoacyl-tRNA substrates which otherwise would be sequestered by the translational machinery

Summary

Introduction

The evolutionary sequence diversity of tRNAs is constrained by the selective pressures to maintain key nucleotides recognized by a variety of cellular factors, such as aminoacyl-tRNA synthetases, translation factors and a variety of other factors that utilize tRNAs for functions beyond that of protein synthesis. As opposed to the previously mentioned example with cell wall biosynthesis in S. aureus, the most optimal tRNA substrate (isoacceptor 5'-CAG-3') for L/F-transferase is the most abundant leucyl-tRNA isoacceptor in E. coli which decodes the most frequently used codon (5'-CUG-3') in a variety of growth rates and media conditions [53,54,55]. Other mechanisms must exist for which L/F-transferase obtains aminoacyl-tRNA substrates such as specific interaction with the aminoacyl tRNA synthetase, which could potentially enable a direct exchange to the tRNA to L/F-transferase or the role of yet unknown cellular factors that could assist in aminoacyl-tRNA binding by L/F-transferase

Bacterial Stringent Response

Stringent Response and the Inactivation of EF-Tu and Translation

Stringent Response and Selective Aminoacylation of tRNA Isoacceptors

Hypothesis

Eukaryotic Aminoacyl-tRNA Protein Transferase and tRNA Substrates

Concluding Remarks