We previously identified three functional elements in the enhancer located downstream of the human immunoglobulin kappa gene. By extending our analysis of site-directed mutations, we were able to characterize the sequence requirements for functional activity of the three motifs. The motif that we designate HM resembles a binding site for Helix-Loop-Helix transcription factors in its sequence requirements within the symmetric consensus element (CANNTG) but appears unique in that it also contains additional sequence requirements 8 bp downstream. Although the second motif (which we designate PU) exactly matched the previously reported mu-B element of the heavy chain enhancer, the sequence requirements for enhancer function were not that of mu-B, but rather resembled the consensus binding site for the ETS family of transcription factors including PU.1. Our analysis of the third element (which we designate DR) has demonstrated important functional nucleotides at the 5' and 3' ends of the element. We hope to purify and characterize the protein(s) binding to this motif. Previously, our transfection studies employed primarily the Burkitt lymphoma line CA46, but we have begun a collaboration with Drs. Jain and Magrath to examine a panel of other Burkitt lines to determine how well the function of several immunoglobulin enhancers are supported in these lines. These studies are designed in part to explore the hypothesis that the unregulated expression of the c-myc oncogene in these cells stems from translocation of this gene into the vicinity of the immunoglobulin enhancers. Current results suggest that, despite the derivation of all of these lines from B lymphocytes expressing immunoglobulin genes, there is a wide variation in the ability of these cells to support function of the various immunoglobulin gene enhancers. We are currently attempting to determine how well the sequence requirements for the human 3' kappa enhancer, as previously studied in CA46, generalize to other Burkitt lines.
