Characterization of a seventh family of immunoglobulin heavy chain VH gene segments in the channel catfish, Ictalurus punctatus.

A VH gene segment that could not be assigned to any of the six known VH gene families of the channel catfish was identified in a genomic clone containing VH gene segments. This gene segment (designated VH7.1) exhibited the structural features characteristic of vertebrate VH genes, specifically potential upstream regulatory sequences, a leader sequence split by an intron, a reading frame that could be readily divided into framework and complementarity determining regions, and a 3' recombination signal sequence. Two regions of nucleotide deletions coupled with degeneracy in the nonamer sequence indicate that this VH gene segment is a pseudogene. Genomic DNA restricted with different enzymes and hybridized under stringent conditions with probes derived from VH7.1 showed that 8-10 bands were present in Southern blots. Reverse transcriptase PCR approaches were used to determine if any of these related sequences were expressed. Sequence analysis of cloned PCR products indicates that different VH gene segments exhibiting > 80% similarity to germline VH7.1 are expressed. Multiple sequence alignments showed that the expressed cVH7a cDNA sequence shared less than 60% nucleotide similarity with representative cDNA sequences from the other known catfish VH gene families. These combined results thus fulfil the criteria for the definition of a new family of catfish VH gene segments. This newly defined, small VH family is designated VH7.