Behavior of tissue culture cells infected with polyoma virus

Baculoviruses and nucleosome management

Negatively-supercoiled-ds DNA molecules, including the genomes of baculoviruses, spontaneously wrap around cores of histones to form nucleosomes when present within eukaryotic nuclei. Hence, nucleosome management should be essential for baculovirus genome replication and temporal regulation of transcription, but this has not been documented. Nucleosome mobilization is the dominion of ATP-dependent chromatin-remodeling complexes. SWI/SNF and INO80, two of the best-studied complexes, as well as chromatin modifier TIP60, all contain actin as a subunit. Retrospective analysis of results of AcMNPV time course experiments wherein actin polymerization was blocked by cytochalasin D drug treatment implicate actin-containing chromatin modifying complexes in decatenating baculovirus genomes, shutting down host transcription, and regulating late and very late phases of viral transcription. Moreover, virus-mediated nuclear localization of actin early during infection may contribute to nucleosome management.

A polyoma mutant that encodes small T antigen but not middle T antigen demonstrates uncoupling of cell surface and cytoskeletal changes associated with cell transformation

The hr-t gene of polyoma virus encodes both the small and middle T (tumor) antigens and exerts pleiotropic effects on cells. By mutating the 3' splice site for middle T mRNA, we have constructed a virus mutant, Py808A, which fails to express middle T but encodes normal small and large T proteins. The mutant failed to induce morphological transformation or growth in soft agar, but did stimulate postconfluent growth of normal cells. Cells infected by Py808A became fully agglutinable by lectins while retaining normal actin cable architecture and normal levels of extracellular fibronectin. These properties of Py808A demonstrated the separability of structural changes at the cell surface from those in the cytoskeleton and extracellular matrix, parameters which have heretofore been linked in the action of the hr-t and other viral oncogenes.

A polyoma mutant that encodes small T antigen but not middle T antigen demonstrates uncoupling of cell surface and cytoskeletal changes associated with cell transformation

The hr-t gene of polyoma virus encodes both the small and middle T (tumor) antigens and exerts pleiotropic effects on cells. By mutating the 3' splice site for middle T mRNA, we have constructed a virus mutant, Py808A, which fails to express middle T but encodes normal small and large T proteins. The mutant failed to induce morphological transformation or growth in soft agar, but did stimulate postconfluent growth of normal cells. Cells infected by Py808A became fully agglutinable by lectins while retaining normal actin cable architecture and normal levels of extracellular fibronectin. These properties of Py808A demonstrated the separability of structural changes at the cell surface from those in the cytoskeleton and extracellular matrix, parameters which have heretofore been linked in the action of the hr-t and other viral oncogenes.

Transforming genes and gene products of polyoma and SV40

The small DNA-containing viruses, SV40 and polyoma, transform cells in vitro and induce tumors in vivo. For both viruses two genes required for transformation have been found. The genes required for transformation are also involved in productive infection. Although the two viruses are similar in their effects on cells, the organization of the transforming genes and gene products is different. The purpose of this review is to compare what is known about the biology and the biochemistry of the early regions of the two viruses. The genetic and biochemical studies defining the sequences important for transformation will be reviewed. Then, the products of the transforming genes, called T antigens, will be discussed in detail. There is a substantial body of descriptive information on those products, and studies on the function of the T antigens have also begun.

The effects of various mammalian sera on attachment efficiency and thymidine incorporation in primary cultures of mouse mammary epithelial cells

Mammary epithelial cells from 16- to 17-day pregnant BALB/c mice were cultured in various mammalian sera to determine the kind of serum which stimulates optimal attachment efficiency and thymidine incorporation. Of those sera tested, horse, bovine, lamb, goat and fetal bovine provided the highest attachment efficiency, whereas rat, mouse and human gave the lowest. Rabbit serum stimulated the highest thymidine incorporation into TCA-insoluble material with goat and rat providing the lowest. These results suggest that sera which provide the highest attachment efficiency for primary cultures are not the best stimulants of DNA synthesis and show that an inverse relationship exists between cell attachment and thymidine incorporation for any given type of mammalian serum.

Nonproductive infection and induction of cellular deoxyribonucleic acid synthesis by bovine adenovirus type 3 in a contact-inhibited mouse cell line

Bovine adenovirus type 3 (BAV-3), which has been reported to produce tumors in newborn hamsters, induced cellular deoxyribonucleic acid (DNA) synthesis in a contact-inhibited mouse kidney cell line (C3H2K). In this system, the virus did not multiply, whereas virus-specific tumor antigen (T antigen) was detected in nearly all cells. Replication of viral DNA could not be detected by DNA-DNA hybridization on membrane filters. The cellular DNA synthesis induced by BAV-3 did occur in the absence of added serum. Extent of induction of cellular DNA synthesis was closely correlated with the multiplicity of infection. Cells activated to synthesize DNA in the serum-free medium by the virus infection progressed to cell division without noticeable cell killing.

Temperature-dependent properties of cells transformed by a thermosensitive mutant of polyoma virus

In cells transformed by ts-3, a thermosensitive mutant of polyoma virus, the loss of inhibition of DNA synthesis by topographical factors (topo-inhibition), is rendered temperature-dependent, providing evidence that the viral genome controls this essential aspect of transformation. The expression of two other attributes of transformed cells, growth in agar and serum requirement for initiation of DNA synthesis in a wound in culture, is not made temperature-dependent. In productive infection of BALB-3T3 cells by ts-3, virus-induced stimulation of cellular DNA synthesis and movement is rendered temperature-dependent.