Regulation of zinc resistance in Campylobacter jejuni

Abstract

Zinc plays a central role in bacteria-host interactions. To control infections, host innate immune defenses exploit the need for bacteria to acquire zinc to proliferate, by restricting availability, as well as the innate toxicity of zinc, by poisoning bacteria with antimicrobial concentrations. Many bacterial pathogens can overcome these defenses via zinc homeostasis proteins and these represent key bacterial virulence factors. Campylobacter jejuni is the leading bacterial cause of human gastroenteritis worldwide yet little is known regarding its virulence factors and their regulation. Previous work identified a metal-resistance operon in C. jejuni NCTC11168 (cj1164c-cj1161c, designated cmr) which contains czcD (alias cj1163c) encoding a member of the Cation Diffusion Facilitator (CDF) family that is required for zinc resistance and associated with C. jejuni virulence. This study aimed to determine the factors involved in CzcD regulation in C. jejuni. CzcD expression was shown to be induced by zinc and cobalt, but not other metals, due to increased transcription from the cmr promoter, Pcmr. In contrast, iron and human-derived iron complexes (hemin, lactoferrin, and transferrin) caused a decrease in CzcD expression which was not associated with altered Pcmr activity but instead was due to sequences located within an upstream untranslated region of czcD. These sequences include the promoter elements for an antisense RNA (asRNA) that is complementary to cj1164c transcripts. Transcription of the asRNA was confirmed and its promoter was shown to be inhibited by iron, with increased asRNA expression associated with enhanced CzcD production. Notably, this study also confirmed that CzcD expression was controlled in response to temperature by an RNA thermometer (RNAT) located within the upstream untranslated region of czcD that sequesters the czcD ribosome binding site to inhibit translation at temperatures below 37°C. This is the first RNAT to be described in C. jejuni and the first to be associated with zinc resistance. The study has uncovered three regulatory elements within the cmr operon that function via different mechanisms to allow differential expression of CzcD in response to temperature, zinc, and iron. It is tempting to speculate that this complex regulation of CzcD provides a mechanism that prevents zinc export in the cooler environments outside of the C. jejuni hosts, presumably where zinc is limited, but that rapidly triggers zinc-export in response to the environmental signals of its warm-blooded hosts where iron is presumed to be limited and zinc-resistance required.

Date of Award	31 Dec 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Jennifer Cavet (Supervisor) & James Linton (Supervisor)