Kinetic and morphological observations on Saccharomyces cerevisiae during spheroplast formation.

A strain of Saccharomyces cerevisiae which produced elongated cells under our growth conditions was investigated. By digestion of the cell walls with snail enzyme, the cells became spheroplasts after a transient state which we termed "prospheroplast." The prospheroplast could be lysed like the spheroplast, but it retained the shape of the original yeast cell if osmotically protected. Prospheroplasts and spheroplasts were prepared, and thin sections of samples taken throughout the process of wall removal were studied in the electron microscope, at regular intervals up to the time of complete conversion to spheroplasts. In addition, cell wall remnants recovered from spheroplast preparations were shadow cast for electron microscopy. This material revealed structures resembling bud scars with attached membranous matter. The kinetic studies showed that after a certain period of time all cells were transformed into prospheroplasts, whereas spheroplast formation started later, depending on the enzyme concentration. In sections, the prospheroplasts appeared to be formed by detachment of the cell walls. Both the prospheroplasts and the spheroplasts showed asymmetric cytoplasmic membranes in which the outer leaflets appeared coated with a dense fibrillar layer. The experiments suggest that, after enzyme digestion, the cytoplasmic membrane retains a coating which is rigid in the prospheroplast but which loses rigidity when the cell is transformed into a spheroplast.