Transcription of three c-myc exons is enhanced in chicken bursal lymphoma cell lines

The chicken c-myc gene, as defined by its homology to the v-myc gene of MC29 virus, is comprised of two exons. Using the techniques of runoff transcription, primer extension, and S1 nuclease protection, we demonstrate that there is a third c-myc exon of approximately equal to 345 base pairs (bp) located 0.7 kbp upstream of the 5' end of the v-myc homology. This first exon is transcribed and present in myc mRNA in normal chicken cells. We also examined RNA from five cell lines derived from avian leukosis virus-induced bursal lymphomas. In all these lines, the level of transcription of the 2.2- to 2.5-kbp myc mRNA is increased 30- to 60-fold over normal cells. The myc mRNA in four of these lines also contains increased levels of the first noncoding exon, and evidence is presented that the long terminal repeat (LTR) in the vicinity of c-myc is functioning as an enhancer of c-myc transcription rather than as a promoter in several of these cell lines. In two cell lines in which the viral LTR has integrated between the first and second exons in the proper orientation for downstream promotion of myc, the LTR does not exhibit promoter function. The pattern of c-myc transcription observed by others in a vast majority of avian leukosis virus-induced neoplasms is not observed in any of the five cell lines examined.