Developmental regulation of gene expression in Tetrahymena.

The onset of gene expression in Tetrahymena thermophila during macronuclear differentiation was investigated by assay of galactokinase in conjugating deoxygalactose-resistant heterokaryons. Our results distinguish three successive states of galactokinase gene expression for cells developing a new macronucleus: stage 0, refractory to induction; stage 1, inducible by refeeding; and stage 2, induced. The refractory period ends at 12 to 13 hr after the onset of conjugation; this corresponds to the time of pair separation, and occurs several hours after the new macronuclei have become morphologically distinguishable. Stage 1 cells behave indistinguishably from mature starved cells. Inhibitor studies suggest that galactokinase synthesis is induced coincidentally with the induction of bulk protein synthesis during conjugation: thus it behaves developmentally like a typical protein; and that galactokinase mRNA is probably transcribed within 1 hr prior to its translation. Thus, when conjugating cells are refed during the refractory period, some developmental condition prevents the swift induction of protein (and galactokinase) synthesis observed upon refeeding starved (nonmating) cells. The possible nature of this developmental phenomenon is discussed.