Studies on polynucleotides: LXXI. Sedimentation and buoyant density studies of some DNA-like polymers with repeating nucleotide sequences

Abstract

The properties of the following DNA-like polymers with repeating sequences were studied by ultraeentrifugation: poly dA:dT, poly dTG:dCA, poly dTC:dGA, poly dTTC:dGAA, poly dTTG:dCAA, poly dTAC:dGTA, poly dATC:dGAT, poly dTATC:dGATA and poly dTTAC:dGTAA. Sedimentation velocity experiments indicated the following molecular weight values for the polymers: poly dA:dT, at least 4 × 10 6; the two repeating dinucleotide polymers (poly dTC:dGA and poly dTG:dCA), approximately 5 × 10 5; the four repeating trinucleotide polymers, approximately 2·5 × 10 5; and the two repeating tetranucleotide polymers, approximately 1 × 10 5. The buoyant densities of the DNA-like polymers were studied by cesium chloride density-gradient centrifugation, The polymers, except for the two repeating tetranucleotide polymers, gave single symmetrical bands by this technique. The two repeating tetranucleotide polymers failed to band in either cesium chloride or cesium sulfate density-gradients. The density values of the DNA-like polymers were not as would be predicted from previously established relations correlating the base composition of naturally occurring DNA's and bouyant density values. Also, heat treatment of the polymers did not cause a uniform increase in the densities, as had been previously found for naturally occurring DNA's; but instead, no density change was found for four polymers, a density decrease was found for poly dTC:dGA and density increases were found for poly dATC:dGAT and poly dTTG:dCAA. The restricted base composition of the DNA-like polymers permitted separation of the individual strands in some cases by centrifugation in alkaline cesium chloride solution. The four single-stranded polydeoxyribonucleotides poly dA, poly dT, poly dTG and poly dAC were isolated by this technique on a preparative scale and their spectral properties are recorded.