1. The rate of protein breakdown was determined on growing and non-growing cultures of thermophilic and mesophilic fungi. 2. In growing cells protein breakdown was negligible. 3. In non-growing cells the breakdown rate of total protein varied between 5.2%/h and 6.7%/h. These values were found to be dependent on both the temperature of the protein breakdown assay and the temperature of growth of the organism. 4. The rate of breakdown of soluble protein in thermophilic fungi was 9-15%/h whereas the rate in mesophilic fungi for the soluble protein fraction was only 4%/h.

6-Phosphogluconate dehydrogenase was isolated and partially purified from the thermophilic fungus Penicillium duponti and the mesophilic fungus Penicillium notatum. The specific activities of the purified enzymes were 17.5 and 22.0 respectively. Optimal activity was obtained at pH 8.0 for both enzymes. Non-linear Arrhenius plots were found for both enzymes with breaks at 30 °C for P. duponti 6-phosphogluconate dehydrogenase and 19 °C for P. notatum 6-phosphogluconate dehydrogenase. The thermal inactivation of 6-phosphogluconate dehydrogenase from both fungi exhibited first order kinetics, and the rate of inactivation for the thermophilic enzyme between 25 °C and 45 °C was only 25% that of the mesophilic enzyme. 6-Phosphogluconate dehydrogenase from both sources was protected from thermal inactivation by 6-phosphogluconic acid, high salt concentration, and high protein concentration. The thermophilic enzyme was found to be more resistant to the denaturants urea, acetamide, and sodium dodecylsulfate.

MILLER, H. M., and M. G. SHEPHERD. 1973. Thermal stability of ribosomes from a thermophilic and a mesophilic fungus. Can. J. Microbiol. 19: 761-763. Ribosomes and ribosomal subunits from the thermophile Peizicilliu/n dripoilti were found to be more thermostable than the corresponding particles from the mesophile Penicilliron notaturn. The thermostability of the ribosomes from both organisms was dependent on nlagnesium ion concentration. The dissociation of the 80-S riboson~es into 60-S and 40-S subunits occi~rred at higher magnesium ion concentrations for the mesophile than the thermophile. MILLER, H. M., et M. G. SHEPHERD. 1973. Thermal stability of ribosomes from a thermophilic and a mesophilic fungus. Can. J. Microbiol. 19: 761-763. On a constate que les ribosomes et les sous-unites de ribosomes obtenus du thermophile Petzicillirim duponti sont plus thermostables que les particules correspondantes obtenues du mesophile Penicilli~mz notatlnn. La thermostabilite des ribosomes provenant des deux organismes depend de la concentration en ions magnesii~n~. La dissociation des ribosomes 80 S en sous-unites 60 S et 40 S, se produit ? i des concentrations plus ClevCes en ions magnesiilm chez le mesophile que chez le thermophlle. [Traduit par le journal]It has been reported previously that the ribosomes of thermophilic bacteria are more heat-stable than the ribosomes of mesophilic bacteria (1-3). No reports have, however, appeared on the thermostability of ribosomes from eucaryotic thermophiles. This study was undertaken to determine the relative heat-stabilitv of ribosomes and ribosomal subunits from a thermophile Peizicillium cluyonti and a mesophile Pei~icillium tzoratun?.Submerged cultures were prepared in the modified Czapek-Dox medium described previously (4) except that the growth in the 5-liter Erlenineyer flasks was in autoclaved liquid medium which did not contain chloramphenicol. Lyophilized cells were ground with alun~ina and suspended in 5 volumes of "Standard buffer" (0.01 M Tris-HC1 buffer pH 7.6, 0.01 M KCl, 0.5 mM dithiothreitol) containing 5 mM MgC12, and crude ribosomes were prepared by differential centrifugation according to the procedure of Nash et al. (5). The crude ribosomes were dialyzed overnight in standard buffer containing 1Received November 24, 1972. whatever concentration of MgCI2 was required in the purified preparations. Purified ribosomes and ribosomal subunits were prepared by centrifuging through 15-30% sucrose gradients at 20 000 rpm for 14.5 h in a Spinco SW 40 rotor at 4°C. The sucrose gradients were prepared in dialysis buffer. The gradients were fractionated and the absorbancy of each fraction was measured at 260 nm. Tube fractions corresponding to pure 80-S ribosomes or 60-S and 40-S subunits were pooled separately and dialyzed overnight against standard buffer containing 2 mM MgC12. Sedimentation coefficients of 80.4 were obtained in the analytical ultracentrifuge for both P. dupoizti and P. notatutlz riboson~es using the method of Taylor and Storck (6).Absorbance-temperatur...