ATP and the processivity of Xenopus laevis DNA polymerase alpha

Azidothymidine triphosphate is an inhibitor of both human immunodeficiency virus type 1 reverse transcriptase and DNA polymerase gamma

The reverse transcriptase from human immunodeficiency virus type 1 was purified from the virus to near homogeneity. The enzyme was shown to possess both RNA-dependent and DNA-dependent DNA-synthesizing activity. Activated DNA as a heteropolymeric substrate was used as efficiently as was the homopolymeric substrate poly(rA)-oligo(dT). The Michaelis-Menten constants were determined for each of the four nucleotides needed to elongate a natural template primer. Azidothymidine triphosphate, a well-known inhibitor of the enzyme, inhibited the enzyme competitively with respect to dTTP and noncompetitively with respect to the other nucleotides. Azidothymidine triphosphate acted as an efficient inhibitor of cellular DNA polymerase gamma, whereas other enzymes of eucaryotic DNA metabolism, namely, DNA polymerase alpha-primase and DNA polymerase beta, were not inhibited. This finding may explain why some acquired immunodeficiency syndrome patients suffer side effects during azidothymidine therapy.

The core region of human glutaminyl-tRNA synthetase homologies with the Escherichia coli and yeast enzymes

We have isolated from a Lambda-gt 11 library a human cDNA clone with one open reading frame of about 2400 bases. A stretch of about 350 amino acids in the deduced amino acid sequence is up to 40 percent identical with parts of the known amino acid sequences of E. coli and yeast glutaminyl (Gln)-tRNA synthetase. The isolated cDNA sequence corresponds to an internal section of a 5500 bases long mRNA that codes for a 170 kDa polypeptide associated with Gln-tRNA synthetase. Thus, the human enzyme is about three times larger than the E. coli and two times larger than the yeast Gln-tRNA synthetase. The three enzymes share an evolutionarily conserved core but differ in amino acid sequences linked to the N-terminal and C-terminal side of the core.

Apparent stimulation of calf thymus DNA polymerase alpha by ATP

The mechanism by which millimolar concentrations of ATP stimulate the activity and increase the processivity of calf thymus DNA polymerase alpha has been investigated with poly(dA)/oligo(dT) as template/primer to eliminate possible effects due to primer synthesis. The effect of ATP on the rate of DNA synthesis with this template/primer was found to be dependent upon whether or not the ATP was neutralized and the species of buffer used in the reaction. The present studies suggest that ATP stimulation of calf thymus DNA polymerase can be attributed to changes in the pH of the reaction mixture, a shift in the magnesium ion optimum, or both. Furthermore, effects of ATP on the processivity of DNA polymerase alpha could be mimicked by lowering the pH of the reaction mixture.

Synthesis of DNA polymerase alpha analyzed by immunoprecipitation from synchronously proliferating cells

Synchronously proliferating TC7 monkey and 3T3 mouse cells were pulse labeled with [35S]methionine. Radioactively labeled DNA polymerase alpha was immunoprecipitated with polymerase-specific monoclonal antibodies. The precipitated polypeptides were identified by gel electrophoresis and fluorography. The increase in DNA polymerase alpha activity during S phase was accompanied by an increased synthesis of the enzyme. Some DNA polymerase alpha was synthesized in growth-arrested TC7 cells whereas the synthesis of the large polymerase subunit in 3T3 cells was strictly coupled to the replicative phase of the cell cycle. We also found that DNA polymerase alpha was more prone to proteolysis in TC7 cells than in 3T3 cells. In 3T3 cells, a polymerase subunit with an apparent molecular weight of 186 000 was observed; this subunit was most probably associated with two smaller subunits of Mr 74 000 and 52 000. Synthesis of these three polymerase-associated polypeptides appeared to be regulated differently.