AN IG HEAVY-CHAIN ENHANCER OF THE CHANNEL CATFISH ICTALURUS-PUNCTATUS- EVOLUTIONARY CONSERVATION OF FUNCTION BUT NOT STRUCTURE

The teleost fishes are among the earliest evolutionary lineages to hav e an Ig heavy chain (IgH) locus whose organization approximates that o f mammals. To understand transcriptional control of the IgH locus in a teleost fish and to gain insight into the evolution of the control el ements, the enhancer activity in the IgH locus of the channel catfish, Ictalurus punctatus, was investigated. Segments of the locus extendin g from upstream of the proximal J(H) gene to 2.5 kb downstream of the second transmembrane (TM2) exon of the mu gene were tested in transien t transfection expression assays in murine myeloma and T cell lines, a nd in catfish B lymphoblastoid, monocytelike, and putative T cell line s. In marked contrast to mammals, no enhancer activity was observed in the catfish J(H) to C mu intron, but strong enhancer activity (approa ching that of the murine IgH intronic enhancer) was identified in a 1. 8-kb segment that included the TM2 exon. This catfish enhancer was act ive in a B lineage-specific manner in both catfish and murine cells. I t was not localized in a small core region, but appeared to contain mu ltiple, dispersed cooperative elements rich in octamer- and mu E5-rela ted motifs. Although the catfish IgH enhancer shares functional charac teristics with the mammalian IgH intronic and 3' enhancers, its unusua l organization does not permit any obvious inferences concerning evolu tionary relationships between the catfish enhancer and any one of the murine IgH enhancers.