Differences in the Use of Cysteine and Glutathione as Sulfur Source in Photoheterotrophic Tobacco Suspension Cultures

Abstract

Photoheterotrophic tobacco cells are able to grow with reduced glutathione (GSH) as sole sulfur source, although growth rate as well as duration of exponential growth are both reduced compared with suspensions grown in sulfate containing medium. Exposure to cysteine as sole sulfur source, however, inhibits the growth of green tobacco suspensions almost completely, and leads to the death of the cells. This effect is not related to an insufficient uptake of cysteine, but rather to an extremely high uptake of this sulfur source from the medium. Therefore, the cysteine mediated growth inhibition does not appear to be due to sulfur starvation, but probably to a disturbed thiol/disulfide - status of the cells. GSH is removed from the medium of tobacco suspensions at a much slower rate than cysteine, which may allow tobacco cells to adapt to this sulfur source before an accumulation of thiol inside the cells causes substantial damage. A time dependent increase in the amount of GSH-sulfur was found in the protein fraction of cell extracts prepared from green tobacco suspensions exposed to [ 35 S]-GSH. This observation indicates that GSH can be utilized by the cells as sulfur source for protein synthesis. In order to make the sulfur moiety of GSH available for protein synthesis it must be assumed that degradation of GSH to its constituent amino acids occurs. However, this degradation seems to take place inside the tobacco cells, and not on the outer membrane as observed in animal tissues: Since GSH is the main labeled, low molecular weight thiol found inside tobacco cells exposed to [ 35 S]-GSH, even in the presence of methionine-S-sul-foximine concentrations known to inhibit de novo synthesis of this peptide almost completely, GSH seems to be taken up by tobacco cells as intact molecule.