OCCURRENCE OF ALTERNATIVELY SPLICED LEADER-DELTA-ONC-POLY(A) TRANSCRIPTS IN CHICKEN NEURORETINA CELLS INFECTED WITH ROUS-ASSOCIATED VIRUS TYPE-1 - IMPLICATION IN TRANSDUCTION OF THE C-MIL C-RAF AND C-RMIL B-RAF ONCOGENES

We previously reported that serial passaging of Rous-associated virus type 1 in nondividing chicken embryo neuroretina cells leads to reprod ucible generation of acutely mitogenic retroviruses that transduced th e catalytic domain of c-mil/c-raf or c-Rmil/B-raf. On the basis of str uctural analysis of several retroviruses, we proposed that the early s tep of oncogene transduction is the constitution of alternatively spli ced leader-DELTAonc-poly(A) transcripts. Here, we show that neuroretin a cells do synthesize hybrid leader-DELTAmil and leader-DELTARmil RNAs and that these RNAs exhibit mitogenic properties and serve as templat es for the generation of transducing retroviruses.