DNA replication-mediated recombination of molecules of adenovirus 2 DNA

Molecules of adenovirus 2 DNA were isolated as nucleoprotein complexes late in infection of HeLa cells and examined by electron microscopy. Some were interpreted as representing intermediates in recombination events. The structures observed were consistent with predicted intermediates according to the model for genetic recombination proposed by Meselson and Radding [Meselson, M. S. & Radding, C. M. (1975) Proc. Natl. Acad. Sci. USA 72, 358-361] and constitute physical evidence from animal cells in support of this model.

Visualization of genetic recombination intermediates of human adenovirus type 2 DNA from infected HeLa cells

The study of recombination in prokaryotes has been facilitated by the availability of recombinational mutants and simple genetic elements such as phages and plasmids. These small but defined molecules of DNA have been especially useful for electron microscopic analysis of structural detail of molecules undergoing recombination both in vivo and in vitro. A limitation in the structural analysis of plasmid recombination is the absolute number of recombining molecules which can be identified and analysed amidst a background of nonrecombining molecules. This limitation would be of even greater consequence in studies of genetic recombination in animal cells. We therefore chose virus-infected animal cells as a model system for the study of the molecular mechanism of genetic recombination in higher organisms. HeLa cells infected with adenovirus serotype 2 (Ad-2) offer several advantages for studying recombination: (1) the virus contains a small and well characterized genome of about 35 kilobases; (2) a large amount of Ad-2 DNA is accumulated during lytic infection and host DNA synthesis is suppressed; (3) Ad-2 recombines at a very high frequency; and (4) similar to phages, animal viruses and Ad-2 in particular are believed to use many of the host cell's enzymes in necessary metabolic processes, presumably including recombination. In this study we used electron microscopic techniques to visualize the structures of in vivo Ad-2 DNA recombination intermediates. Molecules were observed with structures at putative cross-over points which were consistent with the molecular mechanism of recombination proposed by Holliday. In addition, we observed Ad-2 DNA molecules engaged in recombination which were simultaneously serving as templates for replication and/or transcription. To the best of our knowledge, this is the first visualization of in vivo recombination intermediates of discrete DNA molecules isolated from eukaryotic cells.

Repetitive DNA sequences in methotrexate- and methasquin-sensitive and -resistant Chinese hamster cell lines

DNA purified from a Chinese hamster cell line of lung fibroblast origin (DC83F) was analyzed by density gradient centrifugation and by gel electrophoresis after restriction endonuclease digestion in order to fractionate discrete repetitive fractions within the total DNA. No obvious satellite DNAs were resolved using the CsCl or Ag-Cs2SO4 density gradient conditions described herein. However, analysis of the digestion products of a battery of restriction endonucleases indicated that three of these enzymes, EcoR1, HaeIII, and XhoI, yielded discrete fragments which could be visualized with EtBr staining or identified by scintillation counting of [3H]DNA. DNAs from several highly (greater than or equal to hundredfold increased resistance) antifolate-resistant sublines of DC-3F, characterized by a large homogeneously staining region (HSR) in the chromosome complement, were examined with both techniques and compared to the parental, antifolate-sensitive cell line DNA. The density gradient profiles and electrophoretic patterns of restriction endonuclease digests were identical among all the cell lines examined and were indistinguishable from those of the parental DC-3F DNA.