Function
Protein synthesis takes place in cytosolic ribosomes, mitochondria (mitoribosomes), and in plants, the plastids (chloroplast ribosomes). Each of these compartments requires a complete set of functional tRNAs to carry out protein synthesis. The production of mature tRNAs requires processing and modification steps [1] such as the addition of a 3’-terminal cytidine-cytidine-adenosine (CCA). Since no plant tRNA genes encode this particular sequence, a tRNA nucleotidyltransferase must add this sequence post-transcriptionally and therefore is present in all three compartments.
In eukaryotes, multiple forms of tRNA nucleotidyltransferases are synthesized from a single gene and are distributed to different subcellular compartments in the cell. There are multiple in-frame start codons which allow for the production of variant forms of the enzyme containing different targeting information predominantly found in the N-terminal sequence of the protein. [2] In vivo experiments show that the N-terminal sequences are used as transit peptides for import into the mitochondria and plastids. Comparison studies using available tRNA nucleotidyltransferase sequences have identified a single gene coding for this enzyme in plants. Complementation studies in yeast using cDNA derived from Arabidopsis thaliana [3] or Lupinus albus genes [4] demonstrate the biological activity of these enzymes. The enzyme has also been shown to repair damaged or incomplete CCA sequences in yeast. [5]
This enzyme belongs to the family of transferases, specifically those transferring phosphorus-containing nucleotide groups (nucleotidyltransferases).
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