Structure and function of Campylobacter jejuni polynucleotide phosphorylase (PNPase): Insights into the role of this RNase in pathogenicity.

Ribonucleases are in charge of the processing, degradation and quality control of all cellular transcripts, which makes them crucial factors in RNA regulation. This post-transcriptional regulation allows bacteria to promptly react to different stress conditions and growth phase transitions, and also to produce the required virulence factors in pathogenic bacteria. Campylobacter jejuni is the main responsible for human gastroenteritis in the world. In this foodborne pathogen, exoribonuclease PNPase (CjPNP) is essential for low-temperature cell survival, affects the synthesis of proteins involved in virulence and has an important role in swimming, cell adhesion/invasion ability, and chick colonization. Here we report the crystallographic structure of CjPNP, complemented with SAXS, which confirms the characteristic doughnut-shaped trimeric arrangement and evaluates domain arrangement and flexibility. Mutations in highly conserved residues were constructed to access their role in RNA degradation and polymerization. Surprisingly, we found two mutations that altered CjPNP into a protein that is only capable of degrading RNA even in conditions that favour polymerization. These findings will be important to develop new strategies to combat C. jejuni infections.