Growth of rpn11 Mutants under Different Growth Conditions. WT and mutant strains were streaked on YPD at 25, 30 and 37°C. Cells were also plated on complete minimal media containing 1 μg/ml of the amino acid analog canavinine instead of arginine. Plates were photographed after 3–5 days. Three single amino acid substitutions in the MPN+ motif of Rpn11 were studied: his111ala, Ser119ala, and asp122ala. For comparison, a substitution of a highly conserved cysteine residue (cys116ala) that is not part of the MPN+ motif (see Fig. 1) was included as well. At 25°C, all MPN+ mutations are viable but slow growing. Hiss111 is extremely temperature sensitive, showing no appreciable growth even when shifted to 30°C, or in the presence of even 1 μg/ml of the amino acid analog canavinine. Asp122 and Ser119 substitutions show lethality under elevated temperature (37°C) or exposure to canavanine. In comparison, Cys116 substitutions show no growth defects under these conditions.