Minisymposium 24: Membrane transport

Abs # 45002: Membrane proteins involved in ion homeostasis and inorganic carbon uptake of Synechocystis sp. PCC 6803

Presenter:	Wang, HongLiang , hlwang@noble.org
Authors	Wang, HongLiang  (A) (B)  Postier, Bradley L. (A)   Burnap, Robert L. (A)
Affiliations:	(A): Oklahoma State University (B): The Samuel Roberts Noble Foundation
Web Site:	http://microbiology.okstate.edu/faculty/burnap/

Primary ion pumps and antiporters exist as multigene families in Synechocystis sp. PCC 6803. HCO₃^- and CO₂ transport systems also exist in Synechocystis and function to concentrate CO₂ in the vicinity of Rubisco. The CO₂ concentration mechanisms (ccm) are actively involved in Na⁺ and H⁺ movements across membranes and thus factor prominently into overall cellular ion homeostasis. The sll0689 (nhaS3) knockout fails to completely segregate. However, analysis of the partial segregant suggests that this antiporter plays a crucial role in high NaCl tolerance. Single mutants of sll0273 (nhaS2) and slr1595 (nhaS4) exhibit an enhanced fast phase of light-induced H⁺ efflux correlated with pH homeostasis during CO₂ uptake. The double deletions of nhaS2 and nhaS4 almost double the magnitude of the light-driven and Slr1596 (PxcA)-dependent H⁺ efflux, indicating that the gene products act in parallel in vivo. In contrast, pxcA inactivation always leads 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 form the re-entry path of extruded H⁺ rather than the gateway of PxcA-dependent H⁺ efflux. While the loss of nhaS2 causes acute sensitivities to low Na⁺ (~10.0 mM) and alkaline pH (8.0-9.0) during growth under K⁺ replete (~5.3 mM) conditions, these phenotypes do not emerge under lower K⁺ abundances (~0.2 mM). The findings support a hypothesis that NhaS2 is a K⁺(Na⁺)/H⁺ antiporter involved in the K⁺ efflux and alkaline adaptation. On the other hand, the mutation of one copper-trafficking ATPase, sll1920 (pacS), produces a cell line that shows acute NaCl sensitivity as a result of absence of an efficient electron transfer between the cytochrome b₆/f complex and PSI under the stress conditions. Previous work reveals that Sll1594 (NdhR) functions to repress the expressions of slr1727 (nhaS1) and NAD(P)H-dehydrogenase genes crucial for inducible CO₂ uptake (ndhF3/ndhD3) (Figge et al. 2001), implying that the LysR family regulator may exert control over other ion and inorganic carbon translocation systems. DNA microarray analysis reveals that ndhR inactivation up-regulates the expression of sbtA/sbtB, ndhF3/ndhD3/cupA/sll1735, and slr2006-13 including ndhD5 and ndhD6, indicating a vital role of this regulatory gene in both CO₂ and HCO₃^- acquisition of the cyanobacterium. We therefore suggest that ndhR be renamed ccmR to better represent its broader regulatory characteristics.