Isolation and functional characterization of NapA type Na⁺/H⁺ antiporter genes from a halotolerant cyanobacterium Aphanothece halophytica

Abstract

Synechocystis sp. PCC 6803 contains five putative Na⁺/H⁺ antiporters, two homologous to NhaP from Pseudomonas aeruginosa and three homologous to NapA from Enterococcus hirae. Physiological and functional properties of NapA type antiporters are largely unknown. We examined the isolation and characterization of homologous gene from a halotolerant cyanobacterium Aphanothece halophytica. Two genes encoding the same polypeptide size, designated as ApNapA1-1 and ApNapA1-2, were isolated. ApNapA1-1 exhibited higher homology to Synechocystis's one (SynNapA1) than that to ApNapA1-2. ApNapA1-1, ApNapA1-2, and SynNapA1 complemented the Na⁺- and Li⁺-sensitive phenotypes of E. coli mutant. These three antiporters exhibited strongly pH dependent exchange activities, although the activities of ApNapA1-2 were significantly lower than others. Among them, only ApNapA1-2 complemented the K⁺-uptake deficient E. coli mutant and exhibited K⁺-uptake activity. Structural and functional properties of these antiporters were examined and their results will be presented.