No abstract

The myxomycete, Physarum polycephalum , can be induced under laboratory conditions to form two different hard-walled forms, spores and spherules. Characterization of both types of walls revealed only a single sugar, galactosamine. It was identified after acid hydrolysis of the isolated walls by chromatography in three solvent systems, by its positive reaction with ammoniacal silver nitrate, ninhydrin, Galactostat, and the Elson-Morgan test, and by ninhydrin degradation to lyxose. Galactosamine was present as a polymer with solubility characteristics the same as the β1-4–linked glucosamine polymer (chitosan). The walls were also found to contain about 2% protein. Spherule walls revealed a single glycoprotein on gel electrophoresis. Spore walls contained a similar protein component. The phosphate content of isolated spherule walls was 9.8%, and that of spore walls was 1.4%. Spore walls also contained about 15% melanin which was shown to be similar to fungal melanin. A novel method was used to measure the rate of mature spherule formation based on the loss of extractability of P. polycephalum natural pigment. The presence of a rare galactosamine polymer in P. polycephalum spore and spherule walls as the only carbohydrate suggests that the myxomycetes are not closely related to the fungi or the protozoa.

The myxomycetes are called slime molds because of the synthesis of copious amounts of extracellular material (slime) during parts of the life cycle. In Physarum polycephalum , small amounts of slime are produced during exponential growth of microplasmodia in shake flasks, but the amount of this slime increased 10- to 20-fold at 16 to 34 hr after microplasmodia were induced to form spherules by transferring them to salt solution. The slime obtained during both periods is the same; an acidic polysaccharide consisting of galactose, sulfate, and trace amounts of rhamnose. Analysis of the galactose-to-sulfate ratio gave a value of about 4 to 1. Infrared spectroscopy showed increased absorbance at 820 cm −1 characteristic of C-O-S vibrations. Electrophoresis on polyacrylamide gel revealed that the material moved as a single band which stained with Alcian Blue and periodic acid Shiff reagent. However, fractionation of identical material on Dowex columns and electrophoresis on cellulose acetate showed the slime to be made up of three major fractions. The polysaccharide appeared as an extracellular capsule closely adhering to the walls of the spherules. It could be separated from the wall by vigorous shaking. The increased synthesis of slime during spherulation was not blocked by cycloheximide, suggesting that new enzyme synthesis was not necessary for its formation.

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The relationship between protein synthesis and mitosis was studied in Physarum polycephalum, a plasmodial slime mold whose nuclear divisions are synchronous. Results of studies with actidione (cycloheximide), an inhibitor of protein synthesis, indicated that the essential structural proteins for mitosis and nuclear reconstruction were completed prior to the dissolution of the nucleolus in prophase. Proteins that determine the duration of the transition from metaphase to nuclear reconstruction were synthesized from late prophase to prometaphase. It is proposed that these proteins are concerned with the transformation of chemical energy into the mechanical work of mitosis.