INVOLVEMENT OF THE ETS FAMILY FACTOR PU.1 IN THE ACTIVATION OF IMMUNOGLOBULIN PROMOTERS

The B cell-specific expression of immunoglobulin (Ig) genes is control led by the concerted action of variable (V) region promoters and intro nic or 3' enhancers, all of which are active in a lymphoid-specific ma nner, A crucial highly conserved element of the V region promoters is the octamer site -ATTTGCAT-, which can be bound by ubiquitous (Oct-1) as well as B cell-specific (Oct-2) factors. Another less conserved ele ment found in many Ig promoters is pyrimidine-rich and has been shown to be functionally important, in particular for those Ig promoters tha t have only an imperfect octamer site. In this study we have analyzed the factors binding specifically to the pyrimidine-rich motif of the V kappa 19 promoter, a light chain gene promoter with an imperfect octa mer site. Using nuclear extracts prepared from B cells, we detected tw o sets of specific complexes in electrophoretic mobility shift experim ents. One complex appears to be ubiquitous but enriched in lymphoid ce lls and represents the binding of a potentially novel factor with an a pparent molecular mass of similar to 50 kDa. The other complex was fou nd only with extracts from pre-B or B cells as well as from a macropha ge cell Line and appears to be caused by the binding of PU.1, a factor of the Ets family. We show that on this Ig promoter Oct factors (Oct- 1 or Oct-2) and PU.1 can bind concomitantly but without synergism. By transfection experiments in non-B cells we demonstrate that PU.1 is in deed able to activate this promoter in concert with Oct-2. Furthermore , we show that PU.1 can bind with varying affinities to the pyrimidine rich elements of several other Ig promoters. These data suggest a mor e general role for PU.1 or other members of the Ets family in the acti vation of Ig promoters.