Physical state, expression and regulation of two glucocorticoid-controlled genes on bovine papilloma virus vectors

We have studied the extrachromosomal maintenance and the transcription regulation of two glucocorticoid-inducible genes on bovine papilloma virus (BPV) vectors in c127 mouse fibroblasts. These genetic elements were the rat tryptophan oxygenase (TOase) gene promoter, which is active in vivo only in hepatocytes, and the long terminal repeat of the mouse mammary tumor virus (MMTV-LTR). From both genes, fusions of the 5'-flanking region of the transcription unit to the bacterial gene for chloramphenicol acetyltransferase (CATase) were constructed. These fusion genes were inserted either into pCGBPV9, a BPV vector encoding G418 resistance, into pBPV-BV1, a vector containing "stabilizing" segments of the human beta-globin gene, or into a BPV construct, whose bacterial plasmid sequences could be removed before transfection. Five constructs of the two latter groups, selectable in c127 cells only as foci, were normally maintained in the extrachromosomal state. In contrast, three out of five constructs based on pCGBPV9 and selectable for resistance against G418 were maintained in a high molecular weight form, most probably of intrachromosomal concatemeric nature, while the remaining two G418-resistant constructs appeared alternatively in this or the extrachromosomal monomeric form. In contrast to its absence of expression in fibroblasts in vivo, the TOase gene element present on BPV vectors was found to be active in fibroblasts in these transfection experiments. As judged by CATase activities and for TOase also by mapping of the transcription start sites, transcription of both genes was under hormonal regulation. All BPV vectors proved to be useful tools in the study of these regulated genes, and in only one out of ten constructs was regulation atypical, possibly due to effects from flanking vector sequences.