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Transformation of chicken myelomonocytic cells by a retrovirus expressing the v-myb oncogene from the long terminal repeats of avian myeloblastosis virus but not Rous sarcoma virus

To test the effect of long terminal repeat (LTR) regulatory sequences on the transforming capability of the v-myb oncogene from avian myeloblastosis virus (AMV), we have constructed replication-competent avian retroviral vectors with nearly identical structural genes that express v-myb from either AMV or Rous sarcoma virus (RSV) LTRs. After transfection into chicken embryo fibroblasts, virus-containing cell supernatants were used to infect chicken myelomonocytic target cells from preparations of 16-day-old embryonic spleen cells. Both wild-type AMV and the virus expressing v-myb from AMV LTRs (RCAMV-v-myb) were able to transform the splenocyte cultures into a population of immature myelomonocytic cells. The transformed cells expressed the p48v-Myb oncoprotein and formed compact foci when grown in soft agar. In contrast, the virus expressing v-myb from RSV LTRs (RCAS-v-myb) was repeatedly unable to transform the same splenocyte cells, despite being able to infect fibroblasts with high efficiency. This difference in the transforming activities of v-myb-expressing viruses with different LTRs most likely results from the presence of a factor (or factors) within the appropriate myelomonocytic target cell that promotes specific expression from the AMV but not from the RSV LTR.