The ultrastructure of a variant (ρ) form of Mycoplasma is described. The ρ-forms are characterized by dark-ground light microscopy as relatively rigid, unbranched, filamentous organisms with discoidal swellings, and by electron microscopy by the presence of an intracytoplasmic axial fiber extending throughout the length of the cell and associated with a terminal structure of characteristic appearance. In negatively stained preparations the fiber presents a pattern of transverse light and dark major bands, the dark band being divided by a central minor light band. The periodicity of the banding varies from 12.0 to 14.5 nm, and the width of the fiber varies from 40 to 120 nm. The fiber appears to be composed of fibrils aligned parallel to the long axis. The evidence indicates that the fiber contains protein and is devoid of nucleic acid. ρ-Forms were commonly found in Mycoplasma strains derived from goats and occasionally in bovine strains. They may have a wider distribution, as the growth medium was shown to be important both for the expression of the ρ-character and for the selection of the ρ-variant. The functional significance, if any, of the fiber and the terminal structure is unknown.

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The rho-form of Mycoplasma contains a striated, axial fiber and associated terminal structure. The presence of this organelle was correlated with the synthesis of two proteins, A and B, of molecular weights of approximately 85,000 and 26,000, respectively, each accounting for about 10% of the total cell protein. Their amino acid compositions showed them to have distinct polypeptide chains. After osmotic lysis of rho-form cells the organelles disappeared; protein A accompanied the membrane fraction, whereas protein B was partly released in soluble form. After lysis by Nonidet P-40 in a medium composed of 4 M glycerol, 50 mM phosphate, and 10 mM MgSO4 at pH 6 (GPM-6), the organelles were preserved and released with ultrastructure unchanged. Protein A was recovered in the soluble fraction and protein B in the particulate (crude fiber) fraction. Treatment of the crude fiber fraction with 0.5 M NaCl in GPM-6 or with a solution containing 4 M glycerol, 10 mM morpholinoethanesulfonate, and 1 mM ethylenediaminetetraacetate at pH 7.0 caused the fibers to disassemble into subunits. By subsequent changes in the ionic conditions and temperature it was possible to cause the subunits to reassemble into ordered aggregates having the same ultrastructure as the native rho-fibers. The optimum temperature for reassembly in the presence of 4 M glycerol was 37 C, the optimum pH was 6.5 to 7.0, and the presence of Mg-2+, replaceable by Ca-2+, SR-2+, or Ba-2+, was essential. Protein B was the only protein detected in the purified, reconsituted fibers.

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