CHARACTERIZATION OF A 7TH 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 V H gene families of the channel catfish was identified in a genomic clo ne containing VH gene segments. This gene segment (designated VH7.1) e xhibited the structural features characteristic of vertebrate VH genes , specifically potential upstream regulatory sequences, a leader seque nce split by an intron, a reading frame that could be readily divided into framework and complementarity determining regions, and a3' recomb ination signal sequence. Two regions of nucleotide deletions coupled w ith degeneracy in the nonamer sequence indicate that this VH gene segm ent 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 transcr iptase PCR approaches were used to determine if any of these related s equences were expressed. Sequence analysis of cloned PCR products indi cates that different VH gene segments exhibiting >80% similarity to ge rmline VH7.1 are expressed. Multiple sequence alignments showed that t he expressed cVH7a cDNA sequence shared less than 60% nucleotide simil arity 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.