ETS-RELATED PROTEIN PU.1 REGULATES EXPRESSION OF THE IMMUNOGLOBULIN J-CHAIN GENE THROUGH A NOVEL ETS-BINDING ELEMENT

In a primary immune response, a signal from interleukin-2 (IL-2) activates transcription of the gene encoding the pentamer IgM joining component, the J chain. Recently, a bifunctional control element (JB) in the J-chain promoter has been identified. This finding was pursued in the present study by purifying and characterizing the nuclear protein (NF-JB) that mediates the positive regulatory activity of the JB element. The analyses revealed that NF-JB is identical to the Ets-related B-cell- and macrophage-specific transcriptional factor, PU.1, despite the fact that the JB site lacks the GGA core reported to be essential for binding by members of the Ets oncoprotein family. The two factors were found to be indistinguishable with respect to their DNA-binding characteristics, size, and peptide structure. Moreover, in transient transfection assays, PU.1 alone activated reporter constructs containing the JB cis-element, and the activation was shown to be dependent on a glutamine-rich sequence in the amino-terminal portion of PU.1. Finally, a dominant negative mutant of PU.1 was capable of suppressing the transcriptional activity of a 1.2-kb J-chain promoter sequence. These results establish an important role for PU.1 in the regulation of immunoglobulin J-chain gene expression and provide new insights into the function(s) of the Ets transcription factors in lymphoid cells.