Structure and molecular length of the large subunits of RD-114 viral RNA

My Career in Molecular Biology

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▪ Abstract  Norman Davidson’s training as a physical chemist led him to make key early contributions to the chemistry of DNA. He described the details of DNA denaturation and renaturation, concepts that still form the basis for understanding hybridization. He also applied the single-molecule resolution of the electron microscope to describing the chemistry of circular DNA, mapping specific genes, and characterizing heteroduplexes. The latter became a dominant tool for the study of nucleic acids and contributed to our knowledge of transcription, polyadenylation, and retroviral structure. The advent of cDNA cloning and restriction enzymes enabled Davidson to describe the diversity of Drosophila actin genes and to isolate the gene encoding cAMP phosphodiesterase. Davidson then turned his attention to neuroscience and participated in cDNA cloning, oocyte expression, and structure-function studies of nicotinic acetylcholine receptors, voltage-gated sodium channels, a GABA transporter, a G protein-gated potassium channel, and calcium channels. His interests also extended to synaptic plasticity, and he helped to define the role of neuronal nitric oxide synthase and of trkB receptors. His final experiments concerned the role of protein kinase A in long-term potentiation. (The abstract was written posthumously by a colleague.)

Physical properties of moloney murine leukemia virus high-molecular-weight RNA: a two subunit structure

The high-molecular-weight RNA of Moloney murine leukemia virus (MuLV) was analyzed by sedimentation equilibrium ultracentrifugation. Molecular weights of 7.2 x 10(6) and 3.4 x 10(6) were found for the native and subunit forms, respectively, indicating that the native structure is a dimer. S20,w and frictional coefficients were determined for MuLV RNA by analytical velocity centrifugation as a function of ionic strength. The apparent S20,w of native MuLV RNA was 47.3, 57.4, and 66.5 in 0.01, 0.1, and 0.20 M Na+, respectively; the corresponding frictional coefficients were 5.44, 4.48, and 3.87. Native RNA was estimated by circular dichroism to be 85% helical, whereas denatured RNA was 54% helical. Thermal denaturation profiles were obtained from uv absorbance scans. Melting temperatures of 57 and 68 C were obtained for high-molecular-weight RNA in 0.01 M Na+ and 0.122 M Na+, 1mM Mg2+, respectively. van't Hoff plots of the thermal denaturation data gave enthalpies for the helix-coil transition of 21,600 cal (ca. 90,500 J) per mol of cooperatively melting unit in high salt and 19,600 cal (ca. 82,100 J) per mol in low salt, consistent with both base stacking and pairing. The melting of Mu LV RNA occurred over a broad temprange and van't Hoff plots were linear over most of the melting range, indicating a noncooperative process of helix stabilization.

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

The properties and subunit composition of the RNA extracted from RD-114 virions have been 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 5.7 X 10(6) to 7.0 X 10(6). By electron microscopy, under moderately denaturing conditions, the 52S molecule is seen to be an extended single strand with a contour length of about 4.0 mum 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 symmetreically disposed loops on each side of and at equal distance from the center. The 52S molecule consists of two half-size molecules, with molecular weight 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 CH3HgOH 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.