polynucleotide phosphorylase

polynucleotide phosphorylase is a bifunctional enzyme that primarily catalyzes the phosphorolysis of RNA in a reversible, template-independent manner. It was the first enzyme discovered capable of synthesizing ribonucleic acid-like polymers, playing a pivotal role in the early understanding of nucleic acid biosynthesis. While its primary physiological role is in RNA degradation and processing, it has become an indispensable tool in molecular biology for synthesizing homopolymers and studying RNA metabolism.

Function and mechanism

The core activity of this enzyme involves the reversible polymerization of ribonucleoside diphosphates, such as ADP or UDP, into polyadenylic acid or polyuridylic acid, while releasing inorganic phosphate. This phosphorolytic reaction, which proceeds in the 3' to 5' direction, is the reverse of its degradative function, where it breaks down RNA chains. The mechanism is distinct from DNA-dependent RNA polymerase, as it does not require a DNA template. The reaction equilibrium is influenced by the ratio of inorganic phosphate to nucleoside diphosphate, allowing it to function in either synthetic or degradative modes depending on cellular conditions.

Biological role and significance

In prokaryotes like Escherichia coli, the enzyme is a key component of the RNA degradosome, a multi-enzyme complex involved in mRNA turnover and RNA processing. It works in concert with other enzymes like RNase E and the RNA helicase RhlB to regulate gene expression by controlling mRNA stability. Its role is crucial for removing defective transfer RNA molecules and for the quality control of ribosomal RNA. In eukaryotes, a related enzyme, the mitochondrial poly(A) polymerase, shares functional similarities, highlighting its evolutionary significance in organelle RNA metabolism.

Discovery and historical context

The enzyme was discovered in 1955 by Marianne Grunberg-Manago and Severo Ochoa at New York University, a finding for which Severo Ochoa later shared the Nobel Prize in Physiology or Medicine in 1959. Their work with extracts from Azotobacter vinelandii demonstrated the first enzymatic synthesis of an RNA polymer, initially leading to the mistaken belief it was the primary RNA polymerase. This discovery was instrumental in the early days of molecular biology, providing a critical tool for Marshall Nirenberg and Heinrich Matthaei during their Nirenberg and Matthaei experiment that cracked the genetic code using poly-U templates.

Structural characteristics

The enzyme typically forms a multimeric complex, often a trimer of dimers, as seen in structures from Escherichia coli and Streptomyces antibioticus. Its core domain shares structural homology with other nucleotidyltransferase enzymes like RNA polymerase and DNA polymerase I. Key structural features include a central channel that binds the RNA substrate and distinct binding sites for divalent cations like magnesium, which are essential for catalytic activity. The C-terminal region often contains domains for interaction with other components of the RNA degradosome, such as RNase E.

Applications in research and biotechnology

This enzyme has been extensively used to synthesize homopolymeric RNA molecules like poly(A), which are essential for studying translation and mRNA polyadenylation. It was fundamental in the Nirenberg and Leder experiment that elucidated codon assignments. Modern applications include generating RNA ladders for gel electrophoresis, producing templates for in vitro transcription systems, and creating radiolabeled RNA probes for Northern blotting. Its ability to add heteropolymer tails is also utilized in cDNA library construction and next-generation sequencing protocols.

Category:Enzymes Category:Molecular biology