RETROVIRAL TRANSDUCTION OF ONCOGENES - ST EPS AND MECHANISMS

Oncogene transduction, the process by which a cellular gene is capture d by a retrovirus, was mainly described invivo. We have developed a bi ological system allowing stepwise analysis of transduction mechanisms in tissue culture. Avian neuroretina NR) cells, dissected at the 8th d ay of embryonic development, rapidly cease to divide and differentiate in culture. Serial passaging of a retrovirus that does not carry an o ncogene on such cultures, leads with a high frequency to the emergence of new viruses that have transduced oncogenes from the mil/raf family of serine/threonine kinases. These viruses have been selected by thei r ability to induce NR cells division. This experimental system allowe d the isolation of the following molecular intermediates generated dur ing the successive steps of oncogene transduction : a chimeric transcr ipt containing viral and cellular sequences joined together by an alte rnative splicing mechanism ; then a complete retrovirus with a 5' end identical to that of the chimeric RNA ; finally, a retrovirus that has acquired additional gag sequences, and, consequently, and increased r eplicative capacity. Structural analysis of these molecules led us to propose a general model for oncogene transduction in which the key ste p is the synthesis of chimeric RNAs. This model also explains the gene ration of the vast majority of acutely transforming retroviruses isola ted in vivo.