Simple biochemical measurements have been shown to seriously overestimate the production of C-type particles by Chinese hamster ovary (CHO) cells treated with 5-bromodeoxyuridine. First, most particle-bound DNA polymerase activity released by induced cells was associated with particles which had a different density from C-type particles. Second, when labelled with radioactive uridine, induced CHO cells released small amounts of particle-bound radioactivity. Most of the radioactivity, however, was in DNA and did not sediment with the particle-bound polymerase. Thus, few particles which had RNA, an associated DNA polymerase, and the density typical of RNA tumour viruses were released by BrdUrd-induced CHO cells. In spite of this, some immature C-type forms were observed by electron microscopy in partially purified preparations of DNA polymerase-containing particles from induced CHO cells.


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