An IgH enhancer that drives transcription through basic helix-loop-helix and oct transcription factor binding motifs -: Functional analysis of the Eμ3′ enhancer of the catfish

The transcriptional enhancer (E mu3') of the IgH locus of the channel catfish, Ictalurus punctatus, shows strong B cell-specific activity and differs from the mammalian E mu enhancer in both location and structure. It occurs between the mu and delta genes and contains numerous transcription factor binding sites, predominantly octamer and mu E5 motifs of consensus and variant sequences. It lacks the classical muA-mu E3(CBF)-mu beta core array of binding motifs seen within mammalian IgH E mu enhancers. To determine the functionally important motifs, a series of mutant enhancers was created using sequence-targeted polymerase chain reaction. Whereas the mutation of consensus and variant octamer motifs (individually or in multiples) decreased enhancer function, mutation of a single consensus mu E5 motif destroyed the function of this enhancer in mammalian plasmacytomas. Mutation of this consensus mu E5 site, combined with mutations of certain octamer sites, destroyed function in catfish B cells. Experiments using artificial enhancers containing multimers of motifs or short regions of the native enhancer suggested that the minimal E mu3' enhancer (a) contains a consensus mu E5 site and two octamer sites, (b) is B cell-specific, and (c) is active across species. The dependence of an Ig enhancer on sites that bind basic helix-loop-helix and Oct transcription factors has not previously been observed and confirms large differences in structure and function between fish and mammalian IgH enhancers.