Zinc is the second most abundant transition metal in living cells and is involved in numerous biological processes. Regulation of Mycobacterium smegmatis (MSMEG) zinc homeostasis and zinc-specific importers ZnuABC and ZnuABC2 in the mycobacterial cytoplasmic membrane are well described; however, transport across the outer membrane (OM) remained unexplored so far. MspD is a porin highly similar to MspA, the major and well-studied porin responsible for the uptake of several compounds. Unlike MspA and the paralogues MspB and MspC, MspD was found to be transcriptionally induced upon zinc starvation and deletion of the global Z inc U ptake R egulator ZUR. Our study aimed to investigate the role of msp D (msmeg ₆057) in MSMEG zinc homeostasis as well as the regulation of MspD expression. In addition, by growth experiments, quantitative real-time PCR analyses, in situ gene tagging and electron microscopy, we successfully demonstrated the zinc-responsive expression of the MspD protein as well as its complex formation and incorporation into the OM. Overall, the results provided strong evidence for an involvement of the MspD protein in zinc homeostasis of MSMEG.
Goethe et al. (Tue,) studied this question.