  Paper Chase   Cell. Dec 24, 1987;51(6):1001-8. Chung J,
Sussman DJ, Zeller R, Leder P.  Department of Genetics, Harvard
Medical School, Howard Hughes Medical Institute, Boston,
Massachusetts 02115.   The first exon of the c-myc gene has
unusual properties that suggest some further role in gene
regulation. It encodes a large, evolutionarily conserved leader
exon that is transcribed more frequently than the remaining
exons of the c-myc gene. In what follows, we provide a possible
explanation for these observations. We find that the major
promoter of the c-myc gene is bifunctional; that is, it supports
transcription by RNA polymerases II and III (pol II and III).
Both enzymes initiate in vitro transcription from the major
c-myc initiation site (P2), but pol III is completely blocked
near the 3' end of the first exon while pol II, though partially
blocked, transcribes through this region. These superimposed
transcriptional activities suggest a potential regulatory
mechanism by which one polymerase system could