Phenotypic and Genetic Analysis of ``chameleon,'' a Paramecium Mutant with an Enhanced Sensitivity to Magnesium

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RESUMO

Three mutant strains of Paramecium tetraurelia with an enhanced sensitivity to magnesium have been isolated. These new ``Chameleon'' mutants result from partial- or codominant mutations at a single locus, Cha. Whereas the wild type responded to 5 mM Mg(2+) by swimming backward for 10-15 sec, Cha mutants responded with ~30 sec backward swimming. Electrophysiological analysis suggested that this behavior may be caused by slowing in the rate at which a Mg(2+)-specific ion conductance deactivates following membrane excitation. This would be consistent with an observed increase in the sensitivity of Cha mutants to nickel poisoning, since Ni(2+) is also able to enter the cell via this pathway. More extensive behavioral analysis showed that Cha cells also overresponded to Na(+), but there was no evidence for a defect in intracellular Ca(2+) homeostasis that might account for a simultaneous enhancement of both the Mg(2+) and Na(+) conductances. The possibility that the Cha locus may encode a specific regulator of the Mg(2+)- and Na(+)-permeabilities is considered.

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