The intrinsic fluorescence of Tetrahymena as a function of temperature

Abstract

Washed suspensions of Tetrahymena pyriformis GL have an intrinsic fluorescence with an excitation maximum at 291 nm and an emission peaking at 358 nm. These wavelengths are characteristic for protein fluorescence. When suitable optical filters are used to eliminate scattered light, a sharp increase in the fluorescence.is seen on heating the suspensions. The increase in fluorescence at 32°C and peaks at 36°C, with a mid-point at 34°C. The fluorescence-temperature profile does not change during the cell cycle.

On starving the cells in inorganic medium, the fluorescence decreases markedly within a few hours, but reappears when the cells are transferred to a nutrient medium. Slight shifts in the temperature at which maximal fluorescence is seen were noted as cultures grew denser, and large shifts were observed when the culture temperature was altered. These shifts suggest that the conformation of the proteins responsible for the fluorescence emission is in some way determined by the state of the cell membrane. The local anæsthetic dibucaine was able to titrate out the fluorescence, although cytochalasin B and colchicine had no effect, again suggesting that the fluorescence is controlled by the membrane state.

Reversal experiments demonstrated that although the fluorescence was not affected by heating up to 32°C, it was greatly diminished when the cells were heated above 35°C. Damaging the cells by other means, e.g. by brief sonication at low power or by shaking the cell suspensions, also resulted in a decrease in the fluorescence.

Two other species of Tetrahymena, T. vorax V₂S and T. thermophila (Mpr-V) showed similar increases in fluorescence at temperatures a few degrees above their growth temperature. In the latter case, the mid-point of the fluorescence-temperature profile corresponds to the synchrony temperature, as is also the case with T. pyriformis GL.