Rough membranes in Schizosaccharomyces pombe protoplasts

Regenerating protoplasts of Schizosaccharomyces pombe have been examined in the electron microscope after glutaraldehyde fixation. Membrane-bound ribosomes were seen not only on the nuclear envelope and endoplasmic reticulum but also on the outer membranes of mitochondria and on sub-surface cysternae. Functions which have been suggested for sub-surface cysternae in other plant and animal material are considered in relation to this yeast, and the implications for protein synthesis of four different sites for membrane-bound ribosomes are discussed.