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Structure and Genomic Organization of a Second Cluster of Immunoglobulin Heavy Chain Gene Segments in the Channel Catfish^{1 ,2}

Department of Microbiology, University of Mississippi Medical Center, Jackson, MS 39216

The structure, organization, and partial sequence of a 25-kb genomic region containing a second cluster of H chain gene segments in the channel catfish (Ictalurus punctatus) has been determined. Multiple V_H gene segments, representing different V_H families, are located upstream of a germline-joined VDJ. The VDJ segment has a split leader sequence and a single open reading consistent with that expressed in members of the VH1 family. Downstream of the germline-joined VDJ is a single J_H segment and two pseudogene exons structurally similar to the Cµ1 and Cµ2 exons of the functional gene. Both pseudogene exons are multiply crippled with RNA splice sites destroyed, and open reading frames are interrupted by termination codons, insertions, and/or deletions. Sequence alignment of a 10.8-kb region within the second H chain cluster with the genomic sequence of the nine J_H segments and the functional Cµ within the first H chain gene cluster indicates that the second H chain gene cluster probably arose by a massive duplication event. The J_H region of the VDJ, the coding and flanking regions of the single J_H segment, and the pseudogene Cµ exons were readily aligned with homologous segments in the first gene cluster. This duplication event may have extended to include the upstream V_H segments. A member of the Tc1 mariner family of transposable elements is located downstream of the pseudogene Cµ2, which suggests that the transposition may have contributed to the evolution of the duplicated Cµ.

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