Stability & reversibility of adaptation to selenomethionine in Chlorella vulgaris

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Abstract
Resistance to the growth uncoupling effect of selenomethionine, an adaptation which develops in all cells of Chlorella vulgaris, was maintained despite removal of the analogue. Cells that had undergone from 1 to 6 consecutive subcultures with the antimetabolite were subcultured without it and on reexposure were as resistant as before. As many as 22 passages away from selenomethionine, the equivalent of 220 generations, failed to cause deadaptation of cells derived from a population that had previously been cultured 6 times with the analogue. Complete or partial reversal could be achieved at any time, however, if adapted cells were subjected to sulfur starvation, sub-culture with D-methionine as only sulfur source, or subculture with L-methionine as only sulfur source. The reversal data fit the hypothesis that the permanent adaptation to selenomethionine in this alga involves induction of a higher steady state level of the enzymes responsible for reduction of sulfate to methionine.