In vitro infection of lymphoid cells by thymotropic radiation leukemia virus grown in vitro

Murine lymphoid cells were infected in vitro with purified leukemogenic radiation leukemia virus (RadLV) produced by virus-induced lymphoblast cell lines. Thymocytes were shown to be highly susceptible to infection by the virus, whereas murine or other fibroblasts were refractory to it. Murine bone marrow and spleen cells were shown to be much less sensitive to infection by this thymotropic RadLV. By comparison, a B-tropic RadLV isolate (RadLV), propagated on a mouse fibroblast cell line, was noninfectious for lymphoid cells but infected fibroblasts. A correlation was shown to exist between in vitro infection of thymocytes, as assayed by immunofluorescence, and in vivo leukemogenicity of the thymotropic RadLV. This constitutes a rapid in vitro test for in vivo leukemogenicity of a natural lymphatic leukemia virus.

Loss of simian virus 40 DNA-RNA hybrids from nitrocellulose membranes; implications for the study of virus--host DNA interactions

Complete hybrids of simian virus 40 (SV40)DNA and its complementary RNA (cRNA) are not retained on nitrocellulose membranes. At saturating cRNA concentrations, retention of the hybrids indicates incomplete homology between DNA and RNA, probably due to incorporation of host DNA in the viral DNA; this effect is most pronounced when DNA is produced in cells infected at high multiplicity. Hybrids between DNA of Chinese hamster cells transformed by SV40 and cRNA are retained if the DNA fragments are long, but they are lost if the DNA is sheared to less than the length of an SV40 DNA molecule. Hence, in cells examined with about six SV40 genomes per cell, each genome is individually integrated. The results may explain previous discrepancies in the estimation of the number of viral genomes in transformed cells.

Defective bacteriophage PBSH in Bacillus subtilis. I. Induction, purification, and physical properties of the bacteriophage and its deoxyribonucleic acid

PBSH, a defective phage of Bacillus subtilis strain 168, is described. Conditions are given for optimal induction of the prophage with mitomycin C. After a latent period of 90 min, cells were lysed and phage-like particles were released with a burst size of approximately 100 to 400 phage per bacterium. Since no known host supports phage replication after infection, burst size was determined by electron microscope count. Purification procedures and criteria for purity are described. The molecular weight of deoxyribonucleic acid (DNA) extracted from PBSH was estimated by length measurement and sedimentation. No circular DNA molecules were found by either technique. PBSH DNA molecules are linear, double-stranded, and of homogeneous molecular weight, about 12 x 10(6) daltons. There is no evidence for single-strand breaks. The majority of PBSH DNA molecules show a sedimentation behavior dependent on ionic strength. It is inferred that most of the DNA molecules are less hydrodynamically rigid than native DNA having a similar average base composition and molecular weight. Possible reasons for the sedimentation behavior are discussed.

Defective bacteriophage PBSH in Bacillus subtilis. II. Intracellular development of the induced prophage

Treatment of Bacillus subtilis strain 168 with mitomycin C caused induction of a defective prophage, PBSH. During induction, extensive deoxyribonucleic acid (DNA) synthesis took place. Concurrently, a change in marker frequency of the bacterial DNA was noticed. The frequency of only one marker, ade-16, the marker closest to the origin of the bacterial chromosome, was enhanced manyfold. DNA from whole phage particles transformed all bacterial markers at a frequency equal to that of DNA in the noninduced culture, except ade-16, the frequency of which was enhanced 30 to 100 times. Analysis of a double isotope experiment demonstrated that 14% of the phage DNA was derived from preinduction bacterial DNA. The other 86% of DNA in phage particles was DNA replicated after induction. Density label experiments with 5-bromodeoxyuridine showed that postinduction DNA synthesis took place preferentially at the origin region of the bacterial chromosome. Measurement of the molecular weight of DNA replicated after induction clearly showed that postinduction DNA replication is chromosomal. No evidence for prophage detachment and autonomous phage DNA replication was found. The data indicated that, after mitomycin C action, the bacterial chromosome under-went multiple reinitiation at the origin, while normal sequential DNA replication was stopped. The pool of replicated bacterial DNA was fragmented randomly. This DNA was packaged into PBSH particles which were released after cell lysis.