Regulation of potassium uptake in the sodium-resistant (NaCl(r)) and thalium-resistant (TlCl(r)) mutant strain of diazotrophic cyanobacterium Anabaena variabilis

A thalium chloride-resistant (TlCl(r)) mutant strain and a sodium chloride-resistant (NaCl(r)) mutant strain of the diazotrophic cyanobacterium Anabaena variabilis have been isolated by spontaneous and chemical mutagenesis by using TlCl, a potassium (K(+)) analog, and nitrosoguanidine (NTG), respectively. The TlCl(r) mutant strain was found to be defective in K(+) transport and showed resistance against 10 microM TlCl. However, it also showed sensitivity against NaCl (LD(50), 50 m M). In contrast, neither wild-type A. variabilis nor its NaCl(r) mutant strain could survive in the presence of 10 microM TlCl and died even at 1 microM TlCl. The TlCl(r) mutant strain exhibited almost negligible K(+) uptake, indicating the lack of a K(+) uptake system. High K(+) uptake was, however, observed in the NaCl(r) mutant strain, reflecting the presence of an active K(+) uptake system in this strain.DCMU, an inhibitor of PS II, inhibited the K(+) uptake in wild-type A. variabilis and its TlCl(r) and NaCl(r) mutant strains, suggesting that K(+) uptake in these strains is an energy-dependent process and that energy is derived from photophosphorylation. This contention is further supported by the inhibition of K(+) uptake under dark conditions. Furthermore, the inhibition of K(+) uptake by KCN, DNP, and NaN(3) also suggests the involvement of oxidative phosphorylation in the regulation of an active K(+) uptake system. The whole-cell protein profile of wild-type A. variabilis and its TlCl(r) and NaCl(r) mutant strains growing in the presence of 50 m M KCl was made in the presence and absence of NaCl. Lack of transporter proteins in TlCl(r) mutant strain suggests that these proteins are essentially required for the active transport and accumulation of K(+) and make this strain NaCl sensitive. In contrast, strong expression of the transporter proteins in NaCl(r) mutant strain and its weak expression in wild-type A. variabilis is responsible for their resistance and sensitivity to NaCl, respectively. Therefore, it appears that the increased salt tolerance of the NaCl(r) mutant strain was owing to increased K(+) uptake and accumulation, whereas the salt sensitivity of the TlCl(r) mutant strain was owing to the lack of K(+) uptake and accumulation.