ENHANCER COMPLEXES LOCATED DOWNSTREAM OF BOTH HUMAN-IMMUNOGLOBULIN C-ALPHA GENES

To investigate regulation of human immunoglobulin heavy chain expressi on, we have cloned DNA downstream from the two human C alpha genes, co rresponding to the position in the mouse IgH cluster of a locus contro l region (LCR) that includes an enhancer which regulates isotype switc hing. Within 25 kb downstream of both the human immunoglobulin C alpha 1 and C alpha 2 genes we identified several segments of DNA which dis play B lymphoid-specific DNase I hypersensitivity as well as enhancer activity in transient transfections. The corresponding sequences downs tream from each of the two human C alpha genes are nearly identical to each other. These enhancers are also homologous to three regions whic h lie in similar positions downstream from the murine C alpha gene and form the murine LCR. The strongest enhancers in both mouse and human have been designated HS12. Within a 135-bp core homology region, the h uman HS12 enhancers are similar to 90% identical to the murine homolog and include several motifs previously demonstrated to be important fo r function of the murine enhancer; additional segments of high sequenc e conservation suggest the possibility of previously unrecognized func tional motifs. On the other hand, certain functional elements in the m urine enhancer, including a B cell-specific activator protein site, do not appear to be conserved in human HS12. The human homologs of the m urine enhancers designated HS3 and HS4 show lower overall sequence con servation, but for at least two of the functional motifs in the murine HS4 (a kappa B site and an octamer motif) the human HS4 homologs are exactly conserved. An additional hypersensitivity site between human H S3 and HS12 in each human locus displays no enhancer activity on its o wn, but includes a region of high sequence conservation with mouse, su ggesting the possibility of another novel functional element.