Normal isotype switching in B cells lacking the I mu exon splice donor site: evidence for multiple I mu-like germline transcripts.
Ig class switch recombination (CSR) in activated B cells is preceded by the generation of "switch" transcripts from the heavy chain constant region (CH) genes targeted for rearrangement. Switch transcripts include a sterile "I" exon spliced onto the first CH exon. Targeted mutations disrupting the expression or splicing of I exons severely hamper CSR to all tested CH loci, except mu. However, all mu switch transcript mutations tested so far have left the I mu exon splice donor site intact. To test the possibility that the residual CSR activity in I mu mutants could be due to splicing of a truncated I mu exon, we generated new mutants specifically lacking the I mu splice donor site. Surprisingly, normal CSR was observed in the I mu splice donor mutants even in the absence of detectable spliced I mu transcripts. In a search for potential alternative sources of switch-like transcripts in the mu locus, we identified two novel exons which map just upstream of the Smu region and splice onto the C mu 1 exon. Their expression is detectable from early B cell developmental stages, and, at least in hybridomas, it does not require the Emu enhancer. These studies highlight a unique structure for the mu locus I exon region, with multiple nested switch transcript-like exons mapping upstream of Smu. We propose that all of these transcripts directly contribute to mu class switching activity.
Kuzin
I I
II
Department of Medicine, University of Rochester School of Medicine and Dentistry, Rochester, NY 14642, USA.
Ugine
G D
GD
Wu
D
D
Young
F
F
Chen
J
J
Bottaro
A
A
D000818:Animals; D001402:B-Lymphocytes / Q000378:metabolism*; D001483:Base Sequence; D002999:Clone Cells; D003001:Cloning, Molecular; D003433:Crosses, Genetic; D004622:Embryo, Mammalian; D005091:Exons / Q000276:immunology*; D018390:Gene Targeting; D005819:Genetic Markers / Q000276:immunology; D018095:Germ-Line Mutation / Q000276:immunology*; D006825:Hybridomas; D017578:Immunoglobulin Class Switching; D007127:Immunoglobulin Constant Regions / Q000235:genetics; D007074:Immunoglobulin G / Q000235:genetics; D007143:Immunoglobulin Heavy Chains / Q000235:genetics; D007075:Immunoglobulin M / Q000235:genetics / Q000378:metabolism; D007134:Immunoglobulin Switch Region / Q000235:genetics; D007148:Immunoglobulin mu-Chains / Q000235:genetics* / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D008969:Molecular Sequence Data; D012326:RNA Splicing / Q000276:immunology*; D017384:Sequence Deletion / Q000276:immunology*; D013234:Stem Cells; D014158:Transcription, Genetic / Q000276:immunology*
D005819:Genetic Markers; D007127:Immunoglobulin Constant Regions; D007074:Immunoglobulin G; D007143:Immunoglobulin Heavy Chains; D007075:Immunoglobulin M; D007148:Immunoglobulin mu-Chains