Characterization of Ribosomes from the Myxomycete Physarum rigidum Grown in Pure Culture

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The plasmodial phase of the myxomycete Physarum rigidum, analyzed during the period of rapid growth, attained a ribonucleic acid (RNA) and protein content of 9.8 and 60.0%, respectively, on a dry weight basis. It possessed ribosomes of the 80S class which, especially in the absence of magnesium ions, partially dissociated to 60S and 40S subunit classes. Electron micrographs of ribosomes treated with uranyl acetate-lead citrate revealed a number of surface features. Nucleotide analyses of both ribosomal and total RNA disclosed that they were composed of 51.0 and 52.5% guanylic and cytidylic acids, respectively. Consistent with most reports on other organisms, guanylic acid was the most abundant nucleotide found in the various types of RNA and cytidylic acid was the least abundant. The S020,w values of the total RNA classes, in 0.01 sodium acetate (pH 4.6) containing 0.10 m NaCl, were 5.2, 18.1, and 27.3 in S units. Changing the ionic environment of the RNA (0.017 molal potassium phosphate, pH 7.0, containing 0.01 m disodium ethylenediaminetetraacetate) resulted in a reduction of the S020,w values to 4.2, 16.6, and 22.6 in S units, which is indicative of molecular conformational transitions. In general, the amino acid composition of the ribosomal proteins was similar to the data available on ribosomal proteins from other biological sources.

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