Features of the chicken c-myc gene that influence the structure of c-myc RNA in normal cells and bursal lymphomas.

The chicken c-myc gene is the target for proviral insertion mutations in bursal lymphomas and has been transduced to generate several viral oncogenes, but the boundaries of its exons have not been securely established. To define the landmarks of the chicken c-myc gene necessary to produce its mRNA, we used an RNase protection assay and a cDNA clone to analyze the c-myc mRNAs from normal chicken embryos and from two bursal lymphomas: LL6, which contains an avian leukosis virus provirus downstream of the c-myc coding region, and LL7, which contains an avian leukosis virus provirus upstream of the c-myc coding region. Two initiation sites for normal c-myc mRNA are less than 7 bases apart, downstream of a GC-rich region lacking canonical TATA and CAAT sequences. The first exon has two open reading frames for the entire length but no initiator methionine codons. The splice donor and acceptor sites at the boundary of the first intron were assigned by comparing a sequence of an LL6 c-myc cDNA clone with a genomic DNA sequence and confirmed by RNase protection of labeled RNA probes by normal and LL6-derived mRNAs. Two potential polyadenylation signals are located approximately 250 and 400 bases downstream of the c-myc coding region in the third exon, but only the more distal signal is utilized in both normal cells and the LL7 tumor. The proviral integration in the LL6 tumor occurred upstream of the authentic c-myc polyadenylation signal accounting for polyadenylation of this transcript in the proviral long terminal repeat.