Formation and properties of RNA-DNA complexes

Abstract

The rate of formation of complexes between T2 RNA and T2 DNA has been measured at various salt concentrations (0·2 to 1·5 M -KCl) and temperatures (50 to 85°C). With increasing temperature, the rate passes through a maximum which is higher the higher the salt concentration. In 0·5 M -KCI the optimum temperature is 67°C. The rate of reaction is proportional to both the RNA and DNA concentration. The apparent bimolecular constant for five different preparations of T2 RNA was 2 ml./μg DNA/minute, or 10 l./mole nucleotides/second (in 0·5 M -KCI, 67°C, pH 7·3). This rate is several orders of magnitude slower than that for the reaction between polyadenylic acid and polyuridylic acid. Differences and similarities between the two reactions are discussed. Annealing in the absence of RNA causes T2 DNA to lose its RNA-binding ability. The second-order rate constant for this process is approximately the same as that for the RNA-DNA reaction. Under the conditions used for their formation, RNA-DNA complexes are slowly destroyed. The fraction of complex breaking down in unit time increases as the concentration of RNA-DNA complex increases. RNA-DNA complexes were broken down completely by RNase at low salt concentrations. The resistance to RNase increased with increasing salt concentration but was not complete under any condition tested. The complex was destroyed completely by DNase. The extent of complex formation was measured using an excess of either reactant. At least 77% of RNA formed after T2 infection of Escherichia coli is capable of becoming bound to T2 DNA. The binding capacity of T2 DNA is approximately 0·3 μg RNA/μg DNA.