FUNCTIONAL MOTIFS IN THE IGH ENHANCER OF THE CHANNEL CATFISH

The transcriptional enhancer (E mu 3') within the Ig heavy chain (IgH) locus of the channel catfish differs from those found in mammalian Ig H loci in both its location and structure. However, upon transfection into fish or mouse lymphocytes, it activates transcription to an exten t equivalent to that of the mouse IgH intronic enhancer (E mu). Potent ial transcription factor binding motifs in E mu 3' are more numerous t han in mammalian IgH enhancers, and are dispersed over 1.6 kilobases. We transfected catfish and mouse lymphoid cells with reporters under t he control of artificial promoters containing motifs from the catfish enhancer. We demonstrate that 9 of 11 octamer motifs identified in the catfish enhancer, representing five variations of the consensus octam er (ATGCAAAT), are functional in both a catfish B-cell line (1B10) and the mouse plasmacytoma J558L. Only those octamer variants in which on e of the first four bases is altered are active. Clear species differe nces in the strengths of the variant octamer motifs were evident, and in catfish B cells the ATGtAAAT motif was over threefold more active t han the consensus octamer. The one mu A and two mu B motifs in E mu 3' do not contribute to transcriptional activation. These results sugges t that the relative functional contributions of IgH enhancer motifs ha s changed significantly during vertebrate evolution.