Potassium transport in Chlamydomonas reinhardtii: isolation and characterization of transport-deficient mutant strains.

Putative potassium-transport-deficient mutant strains of Chlamydomonas reinhardtii Dang. were induced by ultra-violet mutagenesis and were identified by their dependence on abnormally high concentrations of potassium for growth. Potassium transport studies employing (86)Rb as a tracer were carried out with wild-type cells and with three independently isolated KDP (potassium-dependent phenotype) clones. Wildtype cells exhibit two transport activities. Transport activity A was expressed when cells were grown in medium supplemented with 10 mM KCl. The transporter with type-A activity does not discriminate between either Rb(+) or K(+) as a substrate and has a Km for Rb(+) equal to 1 mM and a Vmax equal to 31 nmol Rb(+) h(-1) 10(-6) cells. Transport activity B was expressed when cells were starved of potassium for 24 h. The transporter with type-B activity prefers K(+) to Rb(+) as a substrate; it has a Km for Rb(+) equal to 2.5 mM and a Vmax equal to 210 nmol Rb(+) h(-1) 10(-6) cells. All three mutant clones exhibit transport activity comparable to type-A when grown in 10 mM KCl. When starved of potassium for 24 h, two KDP clones demonstrate no transport activity and the third clone continues to exhibit only type-A activity.