Regulation of the immunoglobulin heavy chain locus expression at the phylogenetic level of a bony fish: transcription factor interaction with two variant octamer motifs.

Transcriptional control of the IGH locus in teleosts is not fully understood, but evidence from catfish and zebrafish indicates major roles for octamer-binding (Oct) and E-protein transcription factors. A pair of variant octamer motifs in the Emu3' enhancer of the catfish has been shown to be particularly important in driving expression, justifying detailed study of their function. These octamer motifs were examined to determine if they bound Oct2 POU domains in monomeric or dimeric (PORE and MORE) configurations. While catfish Oct2 was shown to be capable of binding PORE and MORE motifs in dimeric conformation, the two octamer motifs in Emu3' bound Oct2 POU domains only in monomeric configuration. Catfish Oct2, when bound in this monomeric conformation, was shown to bend the DNA helix. The relative position of the two octamer motifs in Emu3' affected the activity of the enhancer, and moving the octamer motifs closer together by 5 bp greatly reduced the activity of the enhancer. This effect was not due to steric hindrance preventing the binding of Oct transcription factors to the two motifs, but rather was shown to be due to the disruption of an additional transcription factor binding site lying between the two octamer motifs.