Flow cytometric investigation of heterogeneous copper-sensitivity in asynchronously grown Saccharomyces cerevisiae

The variable stress-sensitivity of individual cells within pure cultures is widely noted but generally unexplained. Here, factors determining the heterogeneous susceptibility to copper toxicity in Saccharomyces cerevisiae were examined with a rapid non-perturbing approach based on flow cytometry. By determination of the DNA content (with propidium iodide) in cell fractions gated by forward angle light scatter (an indicator of the cell volume), it was shown that forward angle light scatter measurements gave an approximation of the cell cycle stage. Thus, our observation that cells in different forward angle light scatter fractions displayed differing Cu-sensitivities indicated that heterogeneous Cu-sensitivity is a function of the cell cycle stage. Furthermore, cells sorted by their Cu-sensitivity and-resistance and subsequently analyzed for DNA content were found predominantly to occupy G1/S and G2/M cell cycle stages, respectively. The oxidant-sensitive probe 2',7'-dichlorodihydrofluorescein diacetate was used to show that the Cu-sensitivity of G2/M phase S. cerevisiae was correlated with greater levels of pre-existing reactive oxygen species in these cells. The results indicate that differential Cu-sensitivity in a S. cerevisiae culture is linked to the cell cycle stage and this link may be determined partly by cell cycle-dependent fluctuations in basal reactive oxygen species generation.