The role of the MarR transcriptional regulators RrpA and RrpB in the response of Campylobacter jejuni to oxidative and aerobic stress.

Campylobacter jejuni is a microaerobic bacterium that possesses complex mechanisms to counter oxidative stress to be able to survive in the presence of reactive oxygen species (ROS). Re-annotation of the C. jejuni NCTC 11168 genome sequence identified two putative MarR-type transcriptional regulators Cj1546 and Cj1556, originally annotated as hypothetical proteins, now designated as RrpA and RrpB (regulator of response to peroxide). Both rrpA and rrpB mutants exhibit increased sensitivity to hydrogen peroxide stress compared to the wild-type strain and both mutants exhibit reduced levels of catalase (KatA) activity. However, neither mutant exhibited any significant difference in sensitivity to either cumene hydroperoxide or menadione oxidative stresses, indicating that RrpA and RrpB do not regulate expression of either alkylhydroperoxide (AhpC) or superoxide dismutase (SodB). rrpA and rrpB mutants exhibit increased biofilm formation, probably due to accumulation of ROS within the cells. Preliminary RNA-seq analysis indicated reduced katA expression in the rrpA mutant, but no differences in katA expression was observed in the rrpB mutant or rrpAB double mutant compared to the wild-type strain. C. jejuni strains normally contain rrpA, whilst only a subset contained rrpB. C. jejuni strains containing both genes were more associated with livestock-associated MLST clonal complexes. The presence of rrpB is linked to a hypervariable region containing the IF subtype of the type I Restriction-Modification (hsd) system, whereas strains containing only rrpA contain the IAB subtype hsd system. Analysis of 43 Brazilian strains identified that most chicken meat isolates contained both genes, whilst most human isolates contained only rrpA. The predominant presence of rrpB in livestock-associated C. jejuni MLST-types suggests an important role for fine-tuning oxidative stress responses through the concerted actions of multiple regulatory proteins in this microaerophilic pathogen. It also highlights the potential of genetic variation in the natural population in the adaptation to different environmental niches.