Polyomavirus minichromosomes: associated DNA topoisomerase II and DNA ligase activities

Association of topoisomerases I and II with the chromatin in SV40-infected monkey cells

The subcellular localization of topoisomerase I and topoisomerase II has been compared in Simian virus (SV40)-infected and uninfected TC7 monkey cells. In SV40-infected cells, both of these enzymes are preferentially associated with the chromatin. Some topoisomerase I is associated with the nuclear matrix, whereas topoisomerase II shows no such association. In uninfected TC7 cells, topoisomerase I is present in both the chromatin and nuclear matrix fractions. Topoisomerase II, on the other hand, is not detected in any of the subcellular fractions of uninfected cells. After SV40 infection, there is a marked increase in the level of chromatin-associated topoisomerase II.

Suppression of BK virus replication and cytopathic effect by inhibitors of prokaryotic DNA gyrase

Nalidixic acid and oxolinic acid, two antibacterial agents known to inhibit bacterial DNA gyrase, are shown to suppress the replication, as well as the cytopathic effect, of BK virus in Vero cell cultures. The inhibition of virus replication was detectable at day 4 post infection in cultures which had been continuously exposed to drugs at concentrations as low as 0.02 to 0.04 mM of nalidixic acid and 0.2 mM of oxolinic acid. These active concentrations are inferior to plasma levels attained in the course of clinical use of the drugs for antibacterial chemotherapy. Also, under these circumstances, no cytotoxicity occurred. The inhibition of development of cytopathology and of virus-induced cell death was demonstrable in cultures treated for 12 days with the drugs. Under these circumstances of prolonged action, oxolinic acid proved to be slightly cytotoxic in that virus inhibitory doses reduced the viability of normal cells. No alterations in the topological conformation of the viral genome or accumulation of end products of viral DNA replication were detected. However, accumulation of viral DNA form I at 48 h post infection suggests that the drugs act through a mechanism involving DNA topoisomerase.

Simian virus 40 minichromosomes contain torsionally strained DNA molecules

Sundin and Varshavsky (J. Mol. Biol. 132:535-546, 1979) found that nearly two-thirds of simian virus 40 (SV40) minichromosomes obtained from nuclei of SV40-infected cells become singly nicked or cleaved across both strands after digestion with staphylococcal nuclease at 0 degrees C. The same treatment of SV40 DNA causes complete digestion rather than the limited cleavages produced in minichromosomal DNA. We have explored this novel behavior of the minichromosome and found that the nuclease sensitivity is dependent upon the topology of the DNA. Thus, if minichromosomes are pretreated with wheat germ DNA topoisomerase I, the minichromosomal DNA is completely resistant to subsequent digestion with staphylococcal nuclease at 0 degrees C. If the minichromosome-associated topoisomerase is removed, virtually all of the minichromosomes are cleaved to nicked or linear structures by the nuclease treatment. The cleavage sites are nonrandomly located; instead they occur at discrete loci throughout the SV40 genome. SV40 minichromosomal DNA is also cleaved to nicked circles and full-length linear fragments after treatment with the single strand-specific endonuclease S1; this cleavage is also inhibited by pretreatment with topoisomerase I. Thus, it may be that the nuclease sensitivity of minichromosomes is due to the transient or permanent unwinding of discrete regions of their DNA. Direct comparisons of the extent of negative supercoiling of native and topoisomerase-treated SV40 minichromosomes revealed that approximately two superhelical turns were removed by the topoisomerase treatment. The loss of these extra negative supercoils from the DNA probably accounts for the resistance of the topoisomerase-treated minichromosomes to the staphylococcal and S1 nucleases. These findings suggest that the DNA in SV40 intranuclear minichromosomes is torsionally strained. The functional significance of this finding is discussed.

An amplified genome that may have resulted from recombination within bidirectionally replicating DNA

Temperature shift-down of permissive mouse cells transformed by a temperature-sensitive (ts) polyomavirus (Py) genome has been shown to induce the accumulation of free copies of the viral DNA. We report here on an unusual product from such induction. The structure of this product is that expected from the occurrence of recombination between growing points in a bidirectionally replicating Py-mouse DNA molecule. This observation may be relevant to the mechanism of gene amplification in mammalian cells.

Simian virus 40 minichromosomes contain torsionally strained DNA molecules

Sundin and Varshavsky (J. Mol. Biol. 132:535-546, 1979) found that nearly two-thirds of simian virus 40 (SV40) minichromosomes obtained from nuclei of SV40-infected cells become singly nicked or cleaved across both strands after digestion with staphylococcal nuclease at 0 degrees C. The same treatment of SV40 DNA causes complete digestion rather than the limited cleavages produced in minichromosomal DNA. We have explored this novel behavior of the minichromosome and found that the nuclease sensitivity is dependent upon the topology of the DNA. Thus, if minichromosomes are pretreated with wheat germ DNA topoisomerase I, the minichromosomal DNA is completely resistant to subsequent digestion with staphylococcal nuclease at 0 degrees C. If the minichromosome-associated topoisomerase is removed, virtually all of the minichromosomes are cleaved to nicked or linear structures by the nuclease treatment. The cleavage sites are nonrandomly located; instead they occur at discrete loci throughout the SV40 genome. SV40 minichromosomal DNA is also cleaved to nicked circles and full-length linear fragments after treatment with the single strand-specific endonuclease S1; this cleavage is also inhibited by pretreatment with topoisomerase I. Thus, it may be that the nuclease sensitivity of minichromosomes is due to the transient or permanent unwinding of discrete regions of their DNA. Direct comparisons of the extent of negative supercoiling of native and topoisomerase-treated SV40 minichromosomes revealed that approximately two superhelical turns were removed by the topoisomerase treatment. The loss of these extra negative supercoils from the DNA probably accounts for the resistance of the topoisomerase-treated minichromosomes to the staphylococcal and S1 nucleases. These findings suggest that the DNA in SV40 intranuclear minichromosomes is torsionally strained. The functional significance of this finding is discussed.