Novel NhaC Na⁺/H⁺ antiporter in cyanobacteria contributes to key molecular processes for salt tolerance

Genome mining has revealed the halotolerant cyanobacterium Halothece sp. PCC7418 harbors considerable enrichment in the ion transport gene family for putative Na⁺/H⁺ antiporters. Here, we compared transcriptomic profiles of these encoding genes under various abiotic stresses and discovered that Halothece NhaC (hnhaC) was one of 24 genes drastically upregulated under salt stress. Critical roles of HnhaC in salt-stress protection and response were identified by a complementation assay using the salt-sensitive mutant Escherichia coli strain TO114. Expression of HnhaC rendered this mutant more tolerant to high concentrations of NaCl and LiCl. Antiporter activity assays showed that HnhaC protein predominantly exhibited Na⁺/H⁺ and Li⁺/H⁺ antiporter activities under neutral or alkaline pH conditions. Furthermore, expression of HnhaC conferred adaptive benefits onto E. coli by enabling a conditional filamentation phenotype. Dissecting the molecular mechanism of this phenotype revealed that differentially expressed genes were associated with clusters of SOS-cell division inhibitor, SOS response repair, and Z-associated proteins. Together, these results strongly indicate that HnhaC is an Na⁺/H⁺ antiporter that contributes to salt tolerance. The ubiquitous existence of several Na⁺/H⁺ antiporters represents a complex molecular system in halotolerant cyanobacteria, which can be deployed differently in response to growth and to environmental stresses.