The IgH locus of the channel catfish, Ictalurus punctatus, contains multiple constant region gene sequences: different genes encode heavy chains of membrane and secreted IgD.
The delta-chain of catfish IgD was initially characterized as a unique chimeric molecule containing a rearranged VDJ spliced to C micro 1, seven C domain-encoding exons (delta1-delta7), and a transmembrane tail. The presence of cDNA forms showing splicing of delta7 to an exon encoding a secretory tail was interpreted to indicate that membrane (deltam) and secreted (deltas) forms were likely expressed from a single gene by alternative RNA processing. Subsequent cloning and sequence analyses have unexpectedly revealed the presence of three delta C region genes, each linked to a micro gene or pseudogene. The first (IGHD1) is located 1.6 kb 3' of the functional C micro (IGHM1). The second (IGHD3) is positioned immediately downstream of a pseudo C micro (IGHM3P), approximately 725 kb 5' of IGHM1. These two delta genes are highly similar in sequence and each contains a tandem duplication of delta2-delta3-delta4. 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 micro pseudogene indicates that the putative deltas product may not be expressed as a chimeric micro delta molecule. Western blots and protein sequencing data indicate that an IGHD3-encoded protein is expressed in catfish serum. Thus, catfish deltam transcripts appear to originate from IGHD1, whereas deltas transcripts originate from IGHD3 rather than, as previously inferred, from a single expressed delta gene. The third delta (IGHD2) is associated with a pseudo C micro (IGHM2P); its presence is inferred by Southern blot analyses.
Bengtén
Eva
E
Department of Microbiology, University of Mississippi Medical Center, Jackson 39216, USA. ebengten@microbio.unsmed.edu
Quiniou
Sylvie M-A
SM
Stuge
Tor B
TB
Katagiri
Takayuki
T
Miller
Norman W
NW
Clem
L William
LW
Warr
Gregory W
GW
Wilson
Melanie
M
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D003001:Cloning, Molecular / Q000379:methods; D020451:Contig Mapping / Q000379:methods; D005803:Genes, Immunoglobulin; D005819:Genetic Markers / Q000276:immunology; D007059:Ictaluridae / Q000235:genetics* / Q000276:immunology*; D007127:Immunoglobulin Constant Regions / Q000737:chemistry / Q000235:genetics*; D007072:Immunoglobulin D / Q000096:biosynthesis / Q000737:chemistry / Q000235:genetics* / Q000302:isolation & purification; D007143:Immunoglobulin Heavy Chains / Q000737:chemistry / Q000235:genetics*; D007138:Immunoglobulin delta-Chains / Q000235:genetics; D007148:Immunoglobulin mu-Chains / Q000235:genetics; D008969:Molecular Sequence Data; D005810:Multigene Family / Q000235:genetics*; D011544:Pseudogenes / Q000276:immunology; D011947:Receptors, Antigen, B-Cell / Q000737:chemistry / Q000235:genetics*
D005819:Genetic Markers; D007127:Immunoglobulin Constant Regions; D007072:Immunoglobulin D; D007143:Immunoglobulin Heavy Chains; D007138:Immunoglobulin delta-Chains; D007148:Immunoglobulin mu-Chains; D011947:Receptors, Antigen, B-Cell