Rates of protein synthesis through the cell cycle of Saccharomyces cerevisiae

Exponentially growing cells of Saccharomyces cerevisiae were fractionated by centrifugation in isotonic, self-generated gradients of Percoll. Rapidly growing cells, μ = 0.5 × h⁻¹, with nearly equal length of the daughter and the parental cell cycle were fractionated according to a cell cycle-related density variation. In these cells the net rate of protein synthesis varies nearly 2-fold during the cell cycle. Subsequent separations according to cell size revealed that the highest rate is observed during G2 period. Slow-growing cells, μ = 0.2 × h⁻¹, were fractionated on shallow Percoll gradients in a bimodal fashion, primarily as a dense daughter fraction and a composite light fraction. Thereby a marked high rate of protein synthesis in large unbudded daughter cells was revealed. Separations according to cell size revealed a cell cycle-related separation of budded cells, and the highest rate is observed, as before, in the G2 period. Irrespective of the growth rate a non-exponential increase of cell protein is thereby observed through the cell cycle of budding yeast. Septation and cell separation coincide with a low degree of ribosome exploitation.