Circular DNA is excised by immunoglobulin class switch recombination.
We have purified extrachromosomal circular DNAs from adult mouse spleen cells, and cloned into a phage vector the BamHl fragments hybridizing with C mu and S gamma 1 probes. We obtained 52 S mu+S gamma 1+ clones by screening 1.4 million phage clones derived from spleen cells stimulated with bacterial lipopolysaccharide and interleukin 4. We have identified the breakpoints of six clones that contain S gamma 1 and S mu sequences fused in the 5' to 3' orientation. All these switch recombination sites were assigned to the central repetitive sequences of the S mu and S gamma 1 regions. Since the common S mu-S gamma 1 sequences at the recombination sites are at most 2 bases long, typical homologous recombination cannot account for their joining. These findings provide direct evidence that mu-gamma 1 class switching can occur by the looping out and excision of chromosomal DNA, with formation of a circle.
Iwasato
T
T
Department of Biophysics, Faculty of Science, Kyoto University, Japan.
Shimizu
A
A
Honjo
T
T
Yamagishi
H
H
D000818:Animals; D001402:B-Lymphocytes / Q000276:immunology; D001483:Base Sequence; D003001:Cloning, Molecular; D004270:DNA, Circular / Q000235:genetics*; D005260:Female; D015723:Gene Library; D005803:Genes, Immunoglobulin; D005809:Genes, Regulator; D005812:Genes, Switch; D005822:Genetic Vectors; D051379:Mice; D008807:Mice, Inbred BALB C; D008969:Molecular Sequence Data; D009693:Nucleic Acid Hybridization; D015345:Oligonucleotide Probes; D010957:Plasmids; D011995:Recombination, Genetic; D015183:Restriction Mapping; D013154:Spleen / Q000276:immunology
D004270:DNA, Circular; D015345:Oligonucleotide Probes