Search for multienzyme complexes of DNA precursor pathways in uninfected mammalian cells and in cells infected with herpes simplex virus type I

Confirmatory evidence for the existence of a multienzyme complex of DNA precursor pathways in mammalian cells was obtained. Using neutral sucrose gradient centrifugation of cell lysates we found that at least five enzymes involved in DNA precursor metabolism in uninfected, S‐phase BHK‐cell fibroblasts cosediment at a common rate, indicative of a multienzyme complex. The enzymes include DNA polymerase thymidine kinase, ribonucleotide reductase, dihydrofolate reductase, and NDP‐kinase. This complex was partially, but not completely, disrupted when lysates from G0‐phase cells were centrifuged. Using lysates from cells infected with herpes simplex virus (HSV) type I some of the virus‐induced ribonucleotide reductase and a minor proportion of the HSV‐thymidine kinase cosedimented rapidly. The virus‐induced DNA polymerase sedimented independently near the middle of the gradient, in contrast to the behaviour of the host polymerase. The enzyme associations observed were disrupted by NaCl or by inclusion of ethylenediamine tetraacetic acid during the cell lysis procedure, instead of the usual EGTA. These results indicate the importance of ionic forces in maintaining the enzyme complexes. The bulk of the DNA and the RNA present in the lysates did not sediment at the same rate as the complexes, showing that the enzymes were not simply adhering nonspecifically to these polyanions. Newly synthesised radiolabeled DNA (15 min pulse with [3H]thymidine) was not preferentially associated with the enzymes, but some functional DNA was evident in the enzyme complex fraction from the uninfected S‐phase cells. DNA polymerase activity in this fraction did not require, nor was it stimulated by, exogenous “activated” DNA. Added DNA primer‐template was required, however, for maximal activity of the polymerase in gradient fractions derived from G0‐phase cells and from HSV‐infected cells. No evidence for channeling of ribonucleotide precursors into DNA of permeabilized cells (uninfected or HSV‐infected) was detected. Most rCDP was incorporated into RNA. In the uninfected, S‐phase cells about 10 pmol/106 cells/90 min of rCDP residues was incorporated into DNA compared with 120 pmol/106 cells/90 min when radiolabeled dCTP was used. Nonradioactive dCTP present in equimolar concentration in the incubation with labeled rCDP did not, however, diminish the incorporation of label from the ribonucleotide. In permeabilized HSV‐infected cells incorporation of radiolabel from rCDP into DNA was ba