Isolation and characterization of magbane, a magnesium-lethal mutant of Paramecium

Discerning the mechanisms responsible for membrane excitation and ionic con trol in Paramecium has been facilitated by the availability of genetic muta nts that are defective in these pathways. Such mutants typically are select ed on the basis of behavioral anomalies or resistance to ions. There have b een few attempts to isolate ion-sensitive strains, despite the insights tha t might be gained from studies of their phenotypes. Here, we report isolati on of "magbane," an ion-sensitive strain that is susceptible to Mg2+. where as the wild type tolerated the addition of greater than or equal to 20 mM M gCl2 to the culture medium before growth was slowed and ultimately suppress ed (at > 40 mM), mgx mutation slowed growth at 10 mM. Genetic analysis indi cated that the phenotype resulted from a recessive single-gene mutation tha t had not been described previously. We additionally noted that a mutant th at was well described previously (restless) is also highly sensitive to Mg2 +. This mutant is characterized by an inability to control membrane potenti al when extracellular K+ concentrations are lowered, due to inappropriate r egulation of a Ca2+-dependent K+ current. However, comparing the mgx and rs t mutant phenotypes suggested that two independent mechanisms might be resp onsible for their Mg2+ lethality. The possibility that mgx mutation may adv ersely affect a transporter that is required for maintaining low intracellu lar Mg2+ is considered.