The activities of the enzymes ornithine transaminase in Saccharomyces cerevisiae and β-galactosidase in Saccharomyces lactis increase periodically once per cell cycle. This step synthesis of the enzymes is observed under either basal or induced conditions; however, when the cells are growing under non-induced conditions, addition of the inducer at any stage of the cell cycle results in an enhancement of the enzyme activity for both enzymes. The amount of enzyme activity induced is constant for much of the cell cycle but doubles over a portion of the cycle, probably as a result of replication of the structural gene of these enzymes during the S phase. These results suggest that whereas there is periodic expression of structural genes during the cell cycle of yeasts grown in a constant environment, these genes can, upon induction, be expressed at any time during the cell cycle.
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β-Galactosidase (EC 22.214.171.124)
ornithine transaminase or L-ornithine:2-oxoacid aminotransferase (EC 126.96.36.199)