Sulphate Uptake in Two Mutants of Chlorella vulgaris with High and Low Sulphur Amino Acid Content

Two mutants of Chlorella vulgaris characterized by higher and lower content of sulphur amino acids compared with the wild strain were assayed for the efficiency of the sulphate uptake mechanism. In both mutants uptake exhibited positive cooperation kinetics and was strongly stimulated by sulphate starvation. Stimulation was depressed by cysteine and to a higher extent by methionine. Mutations affected the uptake efficiency concordantly with the level of sulphur amino acids. Addition to the starved strains of sulphate or chromate reduced the induced transport to the lower values of the non-starved strains. Addition of cycloheximide during the induction period prevented a further enhancement of transport without depressing the attained rate in the low sulphur mutant; it was followed by a rapid fall to the non-induced rate in the high sulphur mutant.