Several mutants resistant to Mn2+ have been isolated and characterized in Saccharomyces cerevisiae. All the mutations are semidominant and allelic to a single nuclear gene (MNRI). Mg2+ in the growth medium reverses the inhibitory effect of Mn2+ in a competitive way. This appears to be due to the inhibition of the uptake of Mn2+ by the cells, not to an increase of the amount of Mg2+ inside the cells. The analysis of the distribution of Mn2+ taken up by growing cells shows that the amount of the ion present in insoluble form is far higher in resistant than in sensitive cells. We therefore believe that yeast cells have a sequestering system for Mn2+ and that the major difference between mutants and wild-type strains lies in the much higher efficiency of this system.
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