Abstract
To express recombinant proteins, we routinely co-transfect DHFR-deficient CHO cells with DNA that encodes a recombinant protein together with a plasmid that encodes the enzyme DHFR. Subsequently, cells producing DHFR are selected and screened for recombinant protein expression. We and others have shown that Chinese hamster ovary (CHO) cells contain several hundred copies of endogenous retrovirus-like sequences that are closely related to mammalian A-type or C-type retroviruses (1,2,3,4). Since both A-type and C-type sequences are actively transcribed in many CHO cell lines(2,3), some elements must reside in transcriptionally active regions of chromatin. Using fluorescence in situ hybridization we have demonstrated that CHO retroviral sequences are distributed randomly among all chromosomes. By using DHFR plasmids containing defective A-type and C-type CHO sequences, we can improve our transfection efficiencies and increase recombinant protein production(5).
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