Human foamy virus can establish persistent infections in human hematopoietic cell lines, such as H92.1.7 (erythroblastoid cells), Jurkat (CD4 ؉ T cells), and U937 (myeloid-monocytic cells). The infection is characterized by constant production of infectious viruses (for >2 1/2 years) with no cytopathic effects on the host cells. Electron microscopy of the infected cells showed a viral morphology similar to that observed for particles produced after acute infection. We have detected, in addition to the full-length form of bel1, a previously described deletion in the bel1 gene of the proviral DNA in these cells. RNA containing this 301-bp deletion, which mapped to the splice donor and acceptor sites of the intron of the bet gene, was also found in encapsidated virion RNA. However, the presence of this defective provirus harboring the deletion in bel1 does not prevent productive persistence in these chronically infected cells, since the virus titer does not decrease during cultivation.

The avian packaging cell line SE21Q1b produces particles which encapsidate cellular RNAs. Such RNAs can be reverse transcribed by endogenous polymerase and integrated into the genomes of newly infected cells (M. Linial, Cell 49:93-102, 1987). Genomic RNA is not packaged because the packaging (⌿) region of the provirus is deleted. The provirus also lacks the negative-strand primer binding site, which prevents efficient reverse transcription of randomly packaged genomic RNA. Previous work from our laboratory suggested that the transacting defect which allows packaging of cellular mRNA mapped to the provirus but did not map to the nucleocapsid region of the gag gene (D.

Using a "single-fly" nucleic acid hybridization method, we have surveyed a collection of D. melanogaster strains in search of variants which affect DNA complementary to the polypyrimidine sequence corresponding to one strand of the 1.705 satellite. Hybridization of labelled polypyrimidine probe to polypurine sequence in nucleic acid extracts of single flies, followed by thermal chromatography over hydroxyapatite led to the identification of one variant. The strain Cy/M(2)S2(10) produced excess hybrid, much of which had low thermal stability. A developmental analysis of the low-melt hybrid phenotype showed that certain tissues, in particular the ovaries, were affected. In addition to the biochemical phenotype, the break down of nurse cell nuclei in Cy/M(2)S2(10) ovaries during oocyte maturation was abnormal. A genetic analysis demonstrated that both the biochemical and cytological phenotypes were the consequences of a single recessive mutation in the DNase-1 gene on chromosome III. Studies with purified DNA demonstrated that the low-melt hybrid phenotype resulted from the accumulation of low molecular weight DNA complementary to the polypyrimidine probe.

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SUMMARYHeat-inactivated routine cytomegalovirus (MCMV) stimulates cellular DNA synthesis in WME, NMG, 3T3, WgIA, chick and NIK-8 cells, but active or u.v.-irradiated MCMV does not. The stimulation of DNA synthesis in NIL-8 and chick cells was studied in detail. We found that both the nuclear and the mitochondrial DNA synthesis were stimulated in these cells. There was no virus DNA synthesis during the period we studied (48 h). The stimulation of nuclear DNA synthesis was about threefold in NIL-8 and 2"5-fold in chick cells as measured by the rate of incorporation of ZH-thymidine (aH-dThd) in the CsC1 fractions which banded at the density of cell DNA. The stimulation was about 9"5-fold in NIL-8 and 1.7-fold in chick cells as detected by autoradiography. There was a 3-fold and a 2.2-fold increase in the degree of incorporation of 3H-dThd into mitochondrial DNA of NIL-8 and chick cells, respectively. The amount of mitochondrial DNA obtained in infected cells of both kinds was about twice that in control cells. The synthesis of mitochondrial DNA was also stimulated by a factor of 2 in the thymidine kinaseless 3T3 cells which incorporate exogenous thymidine exclusively into mitochondrial DNA. There were no MCMV specific antigens detectable by immunofluorescence 5 h after infection, but diffuse nuclear fluorescence could be demonstrated 24 h after infection. Our results indicate that the heat-inactivated virus penetrates the cells, stimulates host DNA synthesis and induces synthesis of early MCMV antigens.