Coexpression of mu and gamma 1 heavy chains can occur by a discontinuous transcription mechanism from the same unrearranged chromosome.

We previously documented that a single BCL1 leukemia cell can produce mu and gamma 1 immunoglobulin heavy chains with identical variable segments in an allelically excluded fashion without heavy chain constant region gene rearrangement. To understand the mechanism of dual mu/gamma 1 synthesis in BCL1 subclones, we have analyzed mature and pre-RNA at the nascent and steady-state levels. We find mu and gamma 1 sequences linked in pre-RNA. However, the primary mu and gamma 1 transcription units are about the same length (approximately 15 kilobases). Initiation of gamma 1 pre-RNA occurs upstream of C gamma 1 at sites identical to those seen in lipopolysaccharide/interleukin-4-induced normal B cells. We propose that dual mu/gamma 1 RNA synthesis occurs by a discontinuous transcription mechanism involving either trans-splicing or ligation of mu pre-RNA initiated 5' of the variable-diversity-joining region to gamma 1 pre-RNA initiated 5' of C gamma 1.