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Polynucleotide phosphorylase, in addition to its role as an exonuclease, is also involved in the post-transcriptional addition of nucleotides to RNA in a template-independent manner:

“Polynucleotide phosphorylase not only synthesizes long, highly heteropolymeric tails in vivo, but also accounts for all of the observed residual polyadenylylation in poly(A) polymerase”

My question is:

What determines which nucleotides are incorporated as the process is template-independent?

I have not been able to find any information about this. Is it purely chance?

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There would seem to be two possibilities:

  1. The enzyme has differential specificity for different substrate NDPs.

or

  1. The deciding factor is the relative concentrations of the different NDPs in the cell.

Although the structure of the enzyme has been determined there do not appear to be any studies that would distinguish between these possibilities. This is perhaps because most recent attention has focussed on the exonuclease activity of the enzyme.

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