Poster: Membrane transport

Abs # 176: In vivo role of antiporters Sll0273 and Slr1595 in pH homeostasis of Synechocystis sp. PCC 6803

Presenter:	Wang, Hong-Liang , hlwang@noble.org
Authors	Wang, Hong-Liang  (A) (B)  Burnap, Robert L. (B)
Affiliations:	(A): Oklahoma State University (B): Current Address: The Samuel Roberts Noble Foundation

Single mutants of the putative Na⁺/H⁺ antiporters, sll0273 (nhaS2) and slr1595 (nhaS4), of the photoautotrophic Synechocystis sp. PCC 6803, exhibit an enhanced fast phase of light-induced H⁺ efflux correlated with pH homeostasis in CO₂ uptake (Wang et al., Mol. Microbiol. 44, 1493-506, 2002). We now report that the double deletions of nhaS2 and nhaS4 almost doubled the activity of the light-driven and Slr1596 (PxcA)-dependent H⁺ efflux in appearance as compared to the single loss of either nhaS2 or nhaS4, indicating that the gene products act in parallel vivo. In contrast, pxcA inactivation always led to an impaired H⁺ extrusion in the genetic background of both single and double mutants of nhaS2 and nhaS4. We therefore propose that NhaS2 and NhaS4 be the re-entry path of extruded H⁺ rather than the gateway of PxcA-dependent H⁺ efflux. RT-PCR revealed that the expression of nhaS4 and its upstream gene, slr1594 (patA), a two-component response regulator, appear to be co-induced by the downshift of Ci (CO₂ and HCO₃^-) availability. While the loss of nhaS2 caused acute sensitivities to low Na⁺ (~10.0 mM) and alkaline pH (8-9) during growth under K⁺ replete (~5.3 mM) conditions, these phenotypes did not emerge under lower K⁺ abundances (~0.2 mM). These findings support the hypothesis that NhaS2 is a K⁺(Na⁺)/H⁺ antiporter involved in the K⁺ efflux and alkaline adaptation in the cyanobacterium.