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The IgH Locus of the Channel Catfish, Ictalurus punctatus, Contains Multiple Constant Region Gene Sequences: Different Genes Encode Heavy Chains of Membrane and Secreted IgD¹

Eva Bengtén^2,*, Sylvie M.-A. Quiniou^3,*, Tor B. Stuge^4,*, Takayuki Katagiri^5,*, Norman W. Miller^*, L. William Clem^*, Gregory W. Warr and Melanie Wilson^*

^* Department of Microbiology, University of Mississippi Medical Center, Jackson, MS 39216; and Department of Biochemistry and Molecular Biology, Medical University of South Carolina, Charleston, SC 29425

The -chain of catfish IgD was initially characterized as a unique chimeric molecule containing a rearranged VDJ spliced to Cµ1, seven C domain-encoding exons (1–7), and a transmembrane tail. The presence of cDNA forms showing splicing of 7 to an exon encoding a secretory tail was interpreted to indicate that membrane (m) and secreted (s) forms were likely expressed from a single gene by alternative RNA processing. Subsequent cloning and sequence analyses have unexpectedly revealed the presence of three C region genes, each linked to a µ gene or pseudogene. The first (IGHD1) is located 1.6 kb 3' of the functional Cµ (IGHM1). The second (IGHD3) is positioned immediately downstream of a pseudo Cµ (IGHM3P), 725 kb 5' of IGHM1. These two genes are highly similar in sequence and each contains a tandem duplication of 2-3-4. However, IGHD1 has a terminal exon encoding the transmembrane region, whereas IGHD3 has a single terminal exon encoding a secreted tail. The occurrence of IGHD3 immediately downstream of a µ pseudogene indicates that the putative s product may not be expressed as a chimeric µ molecule. Western blots and protein sequencing data indicate that an IGHD3-encoded protein is expressed in catfish serum. Thus, catfish m transcripts appear to originate from IGHD1, whereas s transcripts originate from IGHD3 rather than, as previously inferred, from a single expressed gene. The third (IGHD2) is associated with a pseudo Cµ (IGHM2P); its presence is inferred by Southern blot analyses.

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