You are carrying out experiments designed to characterize the structure-function relationships of tRNAs. You isolate an enzyme with tRNA nucleotidyltransferase activity and show that it adds a CCA sequence to the 3′ end of human tRNAs. Which of the following characteristics best distinguishes this enzyme from other RNA polymerases?
A) requires only a GG template
B) synthesizes a specific sequence without a template
C) uses both mRNA and tRNA as substrates
D) uses nucleoside monophosphate as substrates
E) works in a 3′→5′ direction

Question

Quizplus · Accepted Answer

tRNA nucleotidyltransferase adds a CCA sequence to the 3' end of tRNAs without using a template, distinguishing it from other RNA polymerases that require a template to synthesize RNA.  Since all human tRNAs contain variable nucleotide sequence yet also have the CCA sequence added posttranscriptionally, there is no template-specific control for the activity of tRNA nucleotidyltransferase. The growing 3′ terminus of the tRNA progressively refolds to allow the solitary active site of the enzyme to reuse a single CTP-binding site. The ATP-binding site is then created collaboratively by the refolded CC terminus and the enzyme, and nucleotide addition to any tRNA ceases when the nucleotide-binding pocket of the enzyme is full. The template for CCA addition is, therefore, a dynamic ribonucleoprotein structure and not a typical nucleotide template as for other RNA polymerases.

You Are Carrying Out Experiments Designed to Characterize the Structure-Function