Role of Zur-regulated transporters in Mycobacterium avium spp. paratuberculosis during zinc starvation

Zinc homeostasis in bacteria is essential for many biological processes and survival.
Maintenance is achieved by expression of specific systems, e.g. alternative ribosomal
proteins (ARPs) and transporters, which are repressed by the zinc-sensitive global zinc
uptake regulator Zur. Mycobacterium avium ssp. paratuberculosis (MAP) is the causative
agent of Johne's disease in ruminants and suggested to be involved in Crohn’s disease in
humans. MAP and the environmental, nonpathogenic M. smegmatis (MSMEG) possess
three zinc uptake systems, which are induced upon zinc-starvation and/or Zur deletion.
These systems comprise three importers in MAP (znuABCMAP, znuABC-like, mptABC).
MSMEG is equipped with two zinc transporters and a porin (znuABC1, znuABC2, mspD).
The presented study was performed to show whether/how the expression of the zinc
uptake systems is orchestrated in vitro and in vivo and to elucidate the role of Zur in
regulation. Expression of transporters/porins/ARPs was analysed by qRT-PCR after zinc
starvation or after infection of macrophages. In vitro MAP ARP rpmE2 was induced first,
followed by mptABC, znuABC-like and znuABCMAP. rpmE2 and mptABC were also induced
after infection. In MSMEG znuABC1 was induced before znuABC2, rpmG and mspD. β-
galactosidase assay revealed a zinc- and Zur-dependent regulation of znuABCMAP but a zincdependent/
Zur-independent regulation of znuABC-like.
Our data clearly indicate a time and concentration dependent induction of zinc uptake
systems. A different order of induction in MAP
(rpmE2→mptABC→znuABC-like→znuABCMAP) and MSMEG
(znuABC1→znuABC2→rpmG→mspD) suggests diverging zinc response strategies. In
macrophages MAP experiences zinc deficiency. Promoter analyses of znuABC-like indicate
additional regulatory mechanisms for zinc uptake systems in MAP.