"Is Tetrahymena a Plant"?

Is Pneumocystis a plant?

Pneumocystis, an AIDS-associated opportunistic pathogen of the lung has some unusual features. This article focuses on work done by my group to understand the organism's distinct sterols. Although Pneumocystis is closely related to fungi, it lacks the major fungal sterol, ergosterol. Several delta(7) 24-alkysterols synthesized by P. carinii are the same as those reported in some basidiomycete rust fungi. The 24-alkylsterols are synthesized by the action of S-adenosyl-L-methionine:C-24 sterol methyl transferase (SAM:SMT). Fungal SAM:SMT enzymes normally transfer only one methyl group to the C-24 position of the sterol side chain and the cells accumulate C28 24-alkylsterols. In contrast, the P. carinii SAM:SMT and those of some plants catalyze one or two methyl transfer reactions producing both C28 and C29 24-alkylsterols. However, unlike most fungi, plants, and the kinetoplastid flagellates Leishmania and Trypanosoma cruzi, P. carinii does not appear to form double bonds at C-5 of the sterol nucleus and C-22 of the sterol side chain. Furthermore, the P. carinii SAM:SMT substrate preference for C30 lanosterol differs from that of homologous enzymes in any other organisms studied. C31 24-Methylenelanosterol and C32 pneumocysterol, products of SAM:SMT activity on lanosterol, can accumulate in high amounts in some, but not all, human-derived Pneumocystis jiroveci populations.

Environmental effects on Tetrahymena membranes. Temperature-induced changes in membrane fatty acid unsaturation are indepedent of the molecular oxygen concentration

The study was designed to show whether the increase in unsaturated fatty acids in Tetrahymena pyriformis membrane phospholipids with decreasing growth temperature is due to the sharp rise in the concentration of dissolved O2 found at lower temperatures. Increasing cell density in cultures growing at a constant temperature was found to have a more drastic effect upon the O2 level than shifting a culture from one temperature extreme to the other. However, the pattern of phospholipid fatty acid unsaturation did not vary over a 30-fold gradual decrease in O2 concentration measured in cultures during logarithmic growth at 39.5 degrees C, and unsaturation in 15 degrees C cells sampled over a 5-fold decrease in O2 concentration was also unchanged. Cells grown at 39.5 degrees C under a constant relatively high O2 concentration (5--6 mg O2/l) or low O2 concentration (0.3 mg O2/l) had nearly identical distributions of membrane fatty acids. Only under almost completely anaerobic conditions was it possible to measure via radioactive labeling experiments and inhibition of fatty acid desaturation. Thus the increased membrane fatty acid unsaturation induced by decreasing growth temperature is caused by some factor other than the rising levels of dissolved O2.

Dealkylation of 24-ethylsterols by Tetrahymena pyriformis

When Tetrahymena pyriformis was incubated with sitosterol ([24R]-24-ethylcholest-5-en-3 beta-ol]) or its trans-delta22-derivative (stigmasterol), the C-24-dealkylated product, cholesta-5,7,trans-22-trien-3 beta-ol, was obtained in both cases. 24(S)-24-Ethylcholesta-5,7,trans-22-trien-3 beta-ol also was found to be a metabolite. When sitosterol was the substrate, 24(R)-24-ethylcholesta-5,7-dien-3 beta-ol was obtained as a third product. Identifications were made by mass spectroscopy, quantitative chromatography, labeling with 14C, and by other means. The dealkylated product (cholestratrienol) represented 30 percent of the sterols isolable after incubation. The administration of sterols to this organism did not induce sterol biosynthesis, since 2-14C-mevalonate failed to yield radioactive sterol in the presence of added stigmasterol.