Gene identification and functional characterization of a Δ12 fatty acid desaturase in Tetrahymena thermophila and its influence in homeoviscous adaptation to low temperature.

Homeoviscous adaptation in poikilotherms is based in the regulation of the level of desaturation of fatty acids, variation in phospholipids head groups and sterol content in the membrane lipids, in order to maintain the membrane fluidity in response to changes in environmental temperature. Increased proportion of unsaturated fatty acids is thought to be the main response to low-temperature acclimation, which is mostly achieved by fatty acid desaturases. Genome analysis of the ciliate Tetrahymena thermophila and a gene knockout approach has allowed us to identify one Δ12 FAD and to study its activity in the original host and in a yeast heterologous expression system. The "PUFA index" -relative content of polyunsaturated fatty acids compared to the sum of saturated and monounsaturated fatty acid content- was ~57% lower at 15 °C and 35 °C in the Δ12 FAD gene knockout strain (KOΔ12) compared to WT strain. We characterized the role of T. thermophila Δ12 FAD on homeoviscous adaptation and analyzed its involvement in cellular growth, cold stress response, and membrane fluidity, as well as its expression pattern during temperature shifts. Although these alterations allowed normal growth in the KOΔ12 strain at 30 °C or higher temperatures, growth was impaired at temperatures of 20 °C or lower, where homeoviscous adaptation is impaired. These results stress the importance of Δ12 FAD in the regulation of cold adaptation processes, as well as the suitability of T. thermophila as a valuable model to investigate the regulation of membrane lipids and evolutionary conservation and divergence of the underlying mechanisms.

Recent progress in understanding the temporal behavior of unicellular organisms.

This survey summarizes the findings concerning endogenous oscillations of three unicellular organisms: the dinophyte Gonyaulax polyedra, the ciliate Tetrahymena thermophila and the euglenophyte Euglena gracilis. All of them behave rhythmically and show the common features of zeitgeber action, differential sensitivity and temperature compensation; however, they exhibit some species-specific peculiarities that make each of them suitable for addressing particular chronobiological questions. Although ultradian rhythms have been described for Tetrahymena thermophila and Euglena gracilis, they appear under different conditions: in the first case, a modulation of the period in relation to the concentration of nutrients is observed, whereas Euglena oscillates in an ultradian and circadian fashion simultaneously. Transitions between periodic and aperiodic states can be induced in Euglena gracilis and Gonyaulax polyedra: Euglena gracilis can enter an aperiodic state after repeated exposure to short light pulses (up to 10 sec) given at intervals of 40 min or less, whereas in Gonyaulax polyedra the circadian oscillator is arrested at temperatures below 12 degrees C. In the arrhythmic state, the oscillator might be driven into singularity within the phase space of a limit cycle attractor; re-initiation from the holding point occurs by transition to a relatively precisely defined new phase. Photoperiodism as another important chronobiological phenomenon can be studied in Gonyaulax polyedra: cells enter the dormant stage of an asexual cyst under short days and a temperature below 16 degrees C. This response can be mimicked by 5-methoxylated indoleamines such as melatonin and 5-methoxytryptamine, which are synthesized by this organism. Melatonin concentration exhibit an endogenous circadian rhythm characterized by a rapid increase shortly after the onset of darkness. Encystment, as induced by indoleamines, is associated with stimulations of bioluminescence. The coupling of the two processes involves, as a common element, the release of protons from an acidic vacuole.

Recent progress in understanding the temporal behavior of unicellular organisms

This survey summarizes the findings concerning endogenous oscillations of three unicellular organisms: the dinophyte Gonyaulax polyedra, the ciliate Tetrahymena thermophila and the euglenophyte Euglena gracilis. All of them behave rhythmically and show the common features of zeitgeber action, differential sensitivity and temperature compensation; however, they exhibit some species-specific peculiarities that make each of them suitable for addressing particular chronobiological questions. Although ultradian rhythms have been described for Tetrahymena thermophila and Euglena gracilis, they appear under different conditions: in the first case, a modulation of the period in relation to the concentration of nutrients is observed, whereas Euglena oscillates in an ultradian and circadian fashion simultaneously. Transitions between periodic and aperiodic states can be induced in Euglena gracilis and Gonyaulax polyedra: Euglena gracilis can enter an aperiodic state after repeated exposure to short light pulses (up to I 0 sec) given at intervals of 40 min or less, whereas in Gonyaulax polyedra the circadian oscillator is arrested at temperatures below 12 degrees Celsius. In the arrhythmic state, the oscillator might be driven into singularity within the phase space of a limit cycle attractor; re-initiation from the holding point occurs by transition to a relatively precisely defined new phase. Photoperiodism as another important chronobiological phenomenon can be studied in Gonyaulax polyedra: cells enter the dormant stage of an asexual cyst under short days and a temperature below 16 degrees Celsius. This response can be mimicked by 5-methoxylated indoleamines such as melatonin and 5-methoxytryptamine, which are synthesized by this organism. Melatonin concentration exhibits an endogenous circadian rhythm characterized by a rapid increase shortly after the onset of darkness. Encystment, as induced by indoleamines, is associated with stimulations of bioluminescence. The coupling of the two processes involves, as a common element, the release of protons from an acidic vacuole.