Conservation and divergence of the Emicro3' enhancer in the IGH locus of teleosts.

The core region of the Emicro3' transcriptional enhancer that drives the expression of the teleost IGH locus has been characterized functionally in two species, the catfish (Ictalurus punctatus) and the zebrafish (Danio rerio). These studies have suggested important differences: whereas the catfish enhancer acts through an E-box and two octamer motifs, the zebrafish enhancer exerts its major effects through two E-box motifs alone. In this study, the function of the catfish enhancer was reexamined in a broader comparative context within the teleosts. Electrophoretic mobility shift assays of motifs from catfish, zebrafish, and Fugu were conducted to determine their ability to bind catfish E-protein and Oct transcription factors. Transient expression assays were conducted using a region of the catfish core enhancer that includes a newly described hybrid octamer/E-box motif. Sequences homologous to the Emicro3' enhancer region from six teleosts were aligned to determine conserved regions ("phylogenetic footprinting"). These studies allowed the following conclusions to be drawn: (1) The important 3'E-box motif described in the zebrafish corresponds in the homologous region of the catfish enhancer to an Oct motif with a newly described negative regulatory function and (2) Comparison of the Emicro3' enhancer sequences of six teleosts indicates that while a variety of octamer and E-box motifs are found in this region, strict evolutionary conservation of the important functional elements of the teleost Emicro3' enhancer has not occurred.