SUMMARY: Two populations of were isolated from fibroblast tissue cultures and examined for their ability to synthesize DNA when incubated in a defined medium. Both the populations released by mechanical lysis of heavily infected host cells, as well as those recovered from the tissue culture medium, incorporated H PO into DNA. Incorporation occurred at pH 4.5 but not at pH 7.0, and proceeded for 12-15 h. When incorporation of []thymidine was studied, only the organisms obtained by mechanical lysis of host cells were active. Those which had been released by natural means into the tissue culture medium, and then recovered for study, did not incorporate precursor thymidine but were extremely active in protein biosynthesis. In mechanically released organisms, thymidine incorporation was inhibited immediately by rifamycin (40 μM) and hydroxyurea (10 mM), but it was not affected by chloramphenicol (310 μM) until 4 h after addition of the drug. Incorporation of H PO by both populations of organisms was also inhibited by rifamycin, chloramphenicol and hydroxyurea, but the time sequence of inhibition differed. Southern hybridization utilizing P-labelled DNA suggested that both populations synthesized authentic chromosomal DNA sequences, as well as QpH1 plasmid DNA, during acid activation of metabolism.


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