= Polynucleotide phosphorylase
\b[Polynucleotide phosphorylase] is an <enzyme> that can remove <nucleotides> from the $3'$ end of <RNA> by <phosphorolysis>, releasing <nucleoside diphosphates>. Its reversible reaction is conventionally written
$$
\mathrm{RNA}_{n+1}+\mathrm{P_i}
\rightleftharpoons
\mathrm{RNA}_n+\mathrm{NDP},
$$
where $\mathrm{P_i}$ is inorganic <phosphate> and $\mathrm{NDP}$ is the released <nucleoside diphosphate>. The accepted enzyme classification is EC2.7.7.8.<Enzyme Nomenclature by the IUBMB>[EC2.7.7.8, reaction, names and nucleotide donors]{ref}
For a sequence-accounting example, start with $5'$-ACGU-$3'$. One terminal cleavage removes the U unit as <uridine diphosphate>, leaving ACG. Removing the next unit releases the corresponding G-containing diphosphate. The direction is $3'$ toward $5'$; phosphate participation and the diphosphate product distinguish this chemistry from hydrolytic cleavage releasing a <nucleoside monophosphate>.
Under conditions favoring the reverse reaction, <nucleoside diphosphates> can supply units for chain extension. The reversible reaction itself contains no complementary template, so synthetic activity alone does not establish the sequence-copying mechanism of an <RNA polymerase>.<Enzyme Nomenclature by the IUBMB>[EC2.7.7.8, reversible reaction and ADP/IDP/GDP/UDP/CDP donor list]{ref}
A 2000 <X-ray crystallography> study revealed a three-subunit assembly with a central channel. The structure contained a phosphate analogue at catalytic sites but no bound <RNA>. The proposed connection between the channel, RNA handling and <processivity> was therefore a structural model, rather than a direct movie of a chain moving through the enzyme.<A duplicated fold is the structural basis for polynucleotide phosphorylase catalytic activity, processivity, and regulation by M. F. Symmons and others>[Abstract, Results and Conclusions: trimer, tungstate and proposed channel function]{ref}
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