Structure, subunit composition, and molecular weight of RD 114 RNA

The properties and subunit composition of the RNA extracted from RD 114 virions were studied. The RNA extracted from the virion has a sedimentation coefficient of 52S in a nondenaturing aqueous electrolyte. The estimated molecular weight by sedimentation in nondenaturing and weakly denaturing media is in the range of 5.7 x 10 ⁶ to 7.0 x 10 ⁶. By electron microscopy, under moderately denaturing conditions, the 52S molecle is seen to be an extended single strand with a contour length of about 4.0 μm corresponding to a molecular weight of 5.74 x 10 ⁶. It contains two characteristic secondary structure features: (i) a central Y or T shaped structure (the rabbit ears) with a molecular weight of 0.3 x 10 ⁶; (ii) two symmetrically disposed loops on each side of and at equal distance from the center. The 52S molecule consists of two half size molecules, with a molecular weight of 2.8 x 10 ⁶, joined together within the central rabbit ears feature. Melting of the rabbit ears with concomitant dissociation of the 52S molecule into subunits, has been caused by either one of two strongly denaturing treatments: incubation in a mixture of CH ₃HGOH and glyoxal at room temperature, or thermal dissociation in a urea formamide solvent. When half size molecules are quenched from denaturing temperatures, a new off center secondary structure feature termed the branch like structure is seen. The dissociation behavior of the 52S complex and the molecular weight of the subunits have been confirmed by gel electrophoresis studies. The loop structures melt at fairly low temperatures; the dissociation of the 52S molecule into its two subunits occurs at a higher temperature corresponding to a base composition of about 63% guanosine plus cytosine. Polyadenylic acid mapping by electron microscopy shows that the 52S molecule contains two polyadenylic acid segments, one at each end. It thus appears that 52S RD 114 RNA consists of two 2.8 x 10 ⁶ dalton subunits, each with a characteristic secondary structure loop, and joined at the 5' ends to form the rabbit ears secondary structure feature. The observations are consistent with, but do not require, the conclusion that the two 2.8 x 10 ⁶ dalton subunits of 52S RD 114 RNA are identical.