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 reproducible generation of acutely mitogenic retroviruses that transduced the catalytic domain of c-mil/c-raf or c-Rmil/B-raf. On the basis of structural analysis of several retroviruses, we proposed that the early step of oncogene transduction is the constitution of alternatively spliced leader-delta onc-poly(A) transcripts. Here, we show that neuroretina cells do synthesize hybrid leader-delta mil and leader-delta Rmil RNAs and that these RNAs exhibit mitogenic properties and serve as templates for the generation of transducing retorviruses.