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

Abstract

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.

Potassium Fluxes in Chlamydomonas reinhardtii (I.Kinetics and Electrical Potentials)

“…coli (Rhoads et al, 1977). This discrimination against "Rb+ in Chlamydomonas was reported earlier by Polley and Doctor (1985). These authors also observed that the extent of the K+/Rb+ selectivity increased with K+ starvation.…”

“…In a previous study (Polley and Doctor, 1985), K+("Rb+) influx in C. reindavdtii over the range from O to 6 mM was described by a single Michaelis-Menten curve. In the present study, the influx isotherm for K'(4ZK+ and "Rb+) influx was resolved into a biphasic pattern, with saturable and nonsaturable components.…”

“…strain CC125 (mt') was used for a11 experiments. Cells were grown in 6-L culture flasks in TAPM at pH 7.0 (Polley and Doctor, 1985) and synchronized by using a 16-h light/8-h dark regimen. Irradiance was maintained at 200 mE m-'s-' at 25°C within a controlled environment room.…”

“…108, 1995 transport of K+. This organism has been the subject of extensive genetic studies and has provided a convenient system for the isolation of transport mutants (Polley and Doctor, 1985). The studies reported here make use of "Rbt and 42K+ to examine the kinetics of K+ transport over a wide range of [K+],s and to examine changes of K' influx and cellular [K+] associated with K+ deprivation.…”

Selection and Characterization of Chlorella Mutants Deficient in Amino Acid Transport

“…The second approach has so far mainly been used with bacterial and fungal cells (18) and only to some extent with human cells, in the analysis of hereditable diseases related to transport defects (18). Few studies have been made using transport mutants of plant cells (10,19).…”