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Studies on the activity of the A particle-associated Dna polymerase.

The DNA polymerase activity in preparations of A particles from mouse myeloma has been further characterized. Most of the radioactive product from reactions containing poly(rA)·(dT)12–18 and dTTP-3H was composed of dTMP-3H in the form of nuclease-sensitive macromolecules with an average density of 1.428 g/ml, determined in cesium sulfate gradients at neutral pH; the product therefore possessed characteristics of poly(dT). Approximately 50% of this product was similar in size to the template, poly(rA), while the remainder was of lower molecular weight. Studies on the kinetics of the poly(rA)·(dT)12–18-dependent incorporation revealed that enzyme activity was optimal at 37°, with an apparent activation energy of 25.2 kcal/mole, and was higher at the optimal concentration of Mg2+ than with Mn2+, even at Mn2+ concentrations below 0.2 mm. As observed previously, poly(rA)·(dT)12–18 was the most effective template-primer for promoting deoxynucleotide incorporation. However, incorporation was also observed in reactions containing poly(rI)·(dC)12–18 and dCTP-3H, but not in reactions either without template-primer or in those containing the other combinations of deoxynucleotide and template-primers tested. The optimal ratio of µg poly(rA) to µg (dT)12–18 was 6 to 1, and higher levels of (dT)12–18 inhibited dTMP incorporation approximately 50%. In the presence of a 1:1 ratio of poly(rA) to (dT)12–18, highest activity was observed in the range of 0.5 to 6 µg each per reaction.

Studies on the specificity of an enzymatic assay for A particle DNA polymerase revealed that a crude homogenate from myeloma exhibited activity, and mixing of this homogenate with A particles resulted in no change in the activity of the A particle preparation. In contrast, a crude homogenate from A particle-devoid mouse liver possessed no activity, and this homogenate also did not alter the activity of A particles when the two extracts were mixed.