DNA replication in synchronized cultured mammalian cells. V. The temporal order of synthesis of component alpha DNA during monkey DNA synthesis induced by SV40 virus

Infection with the avian polyomavirus, BFDV, selectively affects myofibril structure in embryonic chick ventricle cardiomyocytes

Embryonic cardiomyocytes can both beat and divide. They assemble cardiac muscle-specific proteins into sarcomeric myofibrils and contract. In addition, they periodically synthesize DNA, complete mitosis, disassemble sarcomeric myofibrils in the area of the mitotic spindle, assemble cytoplasmic isoform-specific proteins into a cleavage furrow contractile ring, undergo cytokinesis, and then reform sarcomeric myofibrils in daughter cells. Little is known about how embryonic cardiomyocytes disassemble their myofibrils as they traverse the cell cycle and divide. In the present study, beating embryonic avian ventricular cardiomyocytes in primary culture were stimulated to initiate DNA synthesis without subsequent mitosis or cytokinesis by infection with the lytic avian polyomavirus, Budgerigar Fledgling Disease Virus (BFDV). Within 48 hours, infected, adherent cardiomyocytes disassemble most of their sarcomeric myofibrils, retaining cardiac myosin only in thin myofibrils with disrupted sarcomeric periodicity and in amorphous nonfibrillar pools. By 72 hours, infected cardiomyocytes contain no myofibrils and no longer react with antibodies to cardiac myosin. In contrast, infected cardiomyocytes continue to display cytoplasmic myosin localized in stress-fiber-like-structures in adherent cells, or in disrupted fibers and dispersed pools in detaching cells. Infected cardiomyocytes also continue to display interphase-like arrays of polymerized microtubules, even when rounded-up just prior to lysis. These results suggest that polyomavirus infection may provide a useful model system for further study of the regulation of myofibrils disassembly in embryonic cardiomyocytes.

Effect of simian virus 40 on the temporal and spatial organization of DNA replication in Muntjac cells

We examined the effects of simian virus 40 infection on the temporal and spatial organization of initiation sites for DNA replication in Muntjac cells by means of light microscopic DNA fiber autoradiography. Initiation at multiple sites along the DNA fiber in virus-infected confluent Muntjac cells was more nearly synchronous than in serum-deprived controls, although temporal control in the infected cells did not reach the level observed in cells incubated in serum-enriched medium. Initiation sites in virus-infected cells appeared to be spatially closer together than in either uninfected serum-deprived or uninfected serum-enriched cells. This change did not appear to be the result of the induction or repression by simian virus 40 of clusters of replication units with new and different organizations.

DNA polymerase alpha from the nuclear matrix of cells infected with simian virus 40

The nuclear matrix prepared from normal, simian virus 40 (SV40)-infected, and SV40-transformed cells contained DNA polymerase activities. Approximately 12% of the total DNA polymerase activities in isolated nuclei remained with the nuclear matrix. alpha-polymerase was the major matrix DNA polymerase activity as judged by sensitivity to various inhibitors: aphidicolin, dideoxy-TTP, and N-ethylmaleimide. Approximately 2-4 fold higher DNA polymerase activity was detected in matrices obtained from lytically infected and virus-transformed cells than that found in normal cells. In lytically infected cells, 30-50% of the matrix-bound DNA polymerase activity solubilized by sonication co-sedimented with majority of the matrix T-antigen, and was co-precipitated with anti-T sera. The results suggest that alpha-polymerase and viral T-antigen may form a functional complex in the matrix.

Replication of viral DNA sequences integrated within the chromatin of SV40-transformed Chinese hamster lung cells

To determine when during S phase integrated viral DNA sequences in several tsA SV40-transformed Chinese hamster cell clones replicate, we pulse-labeled cultures with BrdUrd and subsequently collected mitotic cells during sequential time intervals. Restriction endonuclease mapping indicated that each of the three SV40-transformed Chinese hamster lung cell clones contained a single viral DNA sequence at a different, but in each case unique, chromosomal site. DNA was extracted from each population of mitotic cells and then was resolved into dense, BrdUrd-containing and light, unsubstituted DNA fractions by cesium chloride gradient centrifugation. In each pair of samples obtained, we measured viral DNA sequences by solution hybridization using single-stranded SV40 32P-labeled DNA probes. Our results support the conclusions that specific genes within a mammalian DNA are programmed to replicate at particular times during S phase, and that the SV40 A gene product, large T antigen, programs integrated viral DNA sequences to replicate very early in S phase. The fractions of viral DNA replicated early in S phase appeared to correlate with each clone's content of functional large T antigen at permissive and nonpermissive culture temperatures.