Molecular and cellular characterisation of the zinc uptake ( Z nu) system of N ostoc punctiforme

Metal homoeostasis in cyanobacteria is based on uptake and export systems that are controlled by their own regulators. This study characterises the zinc uptake ( Z nu) system in N ostoc punctiforme . The system was found to comprise of three subunits in an ACB operon: a Z n 2+ ‐binding protein ( Z nu A 18), a transmembrane domain ( Z nu B ) and an ATP ase ( Z nu C ). These proteins are encoded within the znu operon regulated by a zinc uptake transcription repressor ( Z ur). Interestingly, a second Z n 2+ ‐binding protein ( Z nu A 08) was also identified at a distal genomic location. Interactions between components of the Z nu ACB system were investigated using knockouts of the individual genes. The znu A 08 − , znu A 18 − , znu B − and znu C − mutants displayed overall reduced znu ACB transcript levels, suggesting that all system components are required for normal expression of znu genes. Zinc uptake assays in the Z n 2+ ‐binding protein mutant strains showed that the disruption of znu A 18 had a greater negative effect on zinc uptake than disruption of znu A 08 . Complementation studies in E scherichia coli indicated that both znu A 08 and znu A 18 were able to restore zinc uptake in a znu A − mutant, with znu A 18 permitting the highest zinc uptake rate. The N . punctiforme zur was also able to complement the E . coli zur − mutant.