Characterization of two cation diffusion facilitators NpunF0707 and NpunF1794 in Nostoc punctiforme

AIMS: To characterize genes involved in maintaining homeostatic levels of zinc in the cyanobacterium Nostoc punctiforme. METHODS AND RESULTS: Metal efflux transporters play a central role in maintaining homeostatic levels of trace elements such as zinc. Sequence analyses of the N. punctiforme genome identified two potential cation diffusion facilitator (CDF) metal efflux transporters, Npun_F0707 (Cdf31) and Npun_F1794 (Cdf33). Deletion of either Cdf31or Cdf33 resulted in increased zinc retention over 3 h. Interestingly, Cdf31(-) and Cdf33(-) mutants showed no change in sensitivity to zinc exposure in comparison with the wild type, suggesting some compensatory capacity for the loss of each other. Using qRT-PCR, a possible interaction was observed between the two cdf's, where the Cdf31(-) mutant had a more profound effect on cdf33 expression than Cdf33(-) did on cdf31. Over-expression of Cdf31 and Cdf33 in ZntA(-) - and ZitB(-) -deficient Escherichia coli revealed function similarities between the ZntA and ZitB of E. coli and the cyanobacterial transporters. CONCLUSIONS: The data presented shed light on the function of two important transporters that regulate zinc homeostasis in N. punctiforme. SIGNIFICANCE AND IMPACT OF THE STUDY: This study shows for the first time the functional characterization of two cyanobacterial zinc efflux proteins belonging to the CDF family.