Electron microscopic analysis of protoplast fusion in Streptomyces hygroscopicus and consideration on structural alterations in fusing membranes

Protoplasts of Streptomyces hygroscopicus were treated with polyethylene glycol and prepared for electron microscopic investigation as ultrathin sections. About 5% binary fusion products and 0.9% multicellular fusion products have been obtained in the sections. Three main types may be differentiated among binary fusion products, characterized by a successive loss of the bispherical shape and of continuous membrane structures in fusion zones.Analysing the membrane alterations a contact zone characterized by intact cytoplasmic membranes in both protoplasts, a fusion zone with a trilaminar fusion membrane of about 13–17 nm in thickness, and a fusion zone without continuous membrane structure can be distinguished. The different fusion areas are considered as stages in the fusion process. The data will be discussed in conjunction with a model for membrane alterations during fusion at the molecular level.