FIG. 3.

Yeast mutants that alter ability to form Mn antioxidants and their effect on oxygen tolerance. Shown are cartoons of four yeast strains, including WT yeast and the corresponding pmr1, smf2, and pho80 mutants that affect the Mn antioxidant. The small balls attached to Mn represent the collection of all the different species of Mn antioxidants, including Mn-Pi as well as other unknown complexes to Mn. PMR1 encodes the Golgi ATPase pump for Mn and in pmr1 mutants, Mn accumulates to very high levels (47). By electron nuclear double resonance spectroscopy, Mn-Pi is 10-fold higher in pmr1 mutants, and these cells are most resistant to oxidative stress (60). SMF2 encodes the Nramp metal transporter for Mn in yeast; smf2 mutants accumulate low Mn and as a result, Mn-Pi levels are ten-fold lower than WT cells (60). The smf2 mutants are more sensitive to oxygen than the WT strains (60). PHO80 encodes the cyclin involved in signaling phosphate control and key stress responses. Loss of pho80 (or pho85) only causes a 2–3-fold reduction in Mn-Pi levels (60), but, nevertheless, Mn can no longer function as an antioxidant in these cells and mutants of pho80 (or pho85) are highly oxygen sensitive. Without SOD enzymes, pho80 (or pho85) mutants cannot survive in air (60, 67). WT, wild-type.