Isolation and Functional Characterization of Ca²⁺/H⁺ antiporters from Cyanobacteria

Abstract

Genome sequences of cyanobacteria revealed the presence of a single Ca²⁺/H⁺ antiporter in these organisms. We isolated the putative Ca²⁺/H⁺ antiporter gene from Synechocystis sp. PCC 6803 (synCAX) as well as a homologous gene from a halotolerant cyanobacterium Aphanothece halophytica (apCAX). In contrast to plant vacuolar CAXs, the full length apCAX and synCAX genes complemented the Ca²⁺-sensitive phenotype of an E. coli mutant. ApCAX and synCAX proteins catalyzed specifically the Ca²⁺/H⁺ exchange reaction at alkaline pH. Immunological analysis suggested their localization in plasma membranes. The Synechocystis sp. PCC 6803 cells disrupted of synCAX exhibited lower Ca²⁺ efflux activity and a salt sensitive phenotype. Overexpression of apCAX and synCAX enhanced the salt tolerance of Synechococcus sp. PCC 7942 cells.