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The Porcine Ig Gene: Unique Chimeric Splicing of the First Constant Region Domain in its Heavy Chain Transcripts ^{1 ,2}

Yaofeng Zhao^3,*, Qiang Pan-Hammarström^*, Imre Kacskovics and Lennart Hammarström^*

^* Center for Biotechnology, Department of Biosciences at Novum, Karolinska Institutet, Huddinge, Sweden; and Department of Physiology and Biochemistry, Faculty of Veterinary Science, Szent István University, Budapest, Hungary

The pig gene is located 3.4 kb downstream of the second transmembrane exon of the µ gene and shows a similar genomic structure to its counterpart in cow with three exons encoding the CH1, CH2, and CH3 domains. The porcine genomic CH1 exon has been replaced by a recent duplication of the µCH1 and its flanking sequences, a genetic event that also led to the formation of a short switch region, immediately upstream of the gene. The CH1 exhibits a 98.7% similarity (314 of 318 bp) to the µCH1 at the DNA level, whereas the homologies between the CH2 and µCH3, and the CH3 and µCH4 are only 33.3 and 35.8%, respectively. Either of the two CH1 exons (µ and ) could be observed in the expressed porcine IgD H chain cDNA sequences VDJ-µCH1-H-CH2-CH3 or VDJ-CH1-H-CH2-CH3, showing a pattern that has not been observed previously in vertebrates. In addition, transfection of a human B cell line, using artificial constructs resembling the porcine Cµ-C locus, also generated both VDJ-µCH1-CH1-H1-CH2 and VDJ -CH1-H1-CH2 transcripts. An examination of the pig genomic sequence shows a putative, second hinge region-encoding exon. Due to the lack of a normal branchpoint sequence for RNA splicing, this exon is not present in the normal pig cDNA. However, the exon could be spliced into most of the expressed transcripts in vitro in cell transfection experiments after introduction of a single T nucleotide to restore the branchpoint sequence upstream of the putative H2 exon.

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