On the nature of the forces involved in the sex-directed flocculation of a fission yeast

Abstract

Cells of Schizosaccharomyces pombe NCYC 132 flocculate during stationary growth phase. Inducible by aeration, flocculation is followed by conjugation and formation of ascospores. Not all cells in a culture are competent to form flocs. Cells in flocs are separable from the in competent cells by differential sedimentation. Stable when diluted in deionized water, flocs are reversibly deflocculated by heat. Tm, the temperature at which half a floe population is deflocculated, is 61 °C in deionized water. Mono- and di-valent cations increase Tm and the rate of reflocculation at ambient temperature. Guanidinium chloride, urea, sodium dodecyl sulfate, extremes of pH, and ultrasonication also reversibly deflocculate purified flocs. In contrast, proteinases, e.g., pronase and trypsin, cause irreversible deflocculation. All reflocculation exhibits hysteresis. It is very likely that the principal binding forces are hydrogen and hydrophobic bonds between walls of competent cells, rather than ionic bonds. The data also suggest the involvement of some proteinaceous structure on the cell wall.