THE PU.1 BINDING-SITE IS A CIS-ELEMENT THAT REGULATES PRO-B PRE-B SPECIFICITY OF V-KAPPA-J-KAPPA JOINING/

We have previously shown that the PU.1 binding motif (GAG CAA) in the 3'-enhancer region in the Ig kappa gene is responsible for the negativ e regulation of tissue (B/T)-specific V kappa-J kappa joining, Here we report that the PU.1 binding site also regulates the stage (pro-B/pre -B) specificity of V kappa-J kappa joining, In the substrate with base substitutions in the PU.1 binding motif, recombination took place in bath pro-B (B220(dull)/CD43(+)) and pre-B (B220(dull)/CD43(-)) cells, In the transcriptional regulation, the PU.1 motif acts in a positive m anner cooperatively with the nuclear factor-EMS (or PIP) motif (GAAAAC ), which is located 2 bp downstream from the PU.1 motif, Interestingly , base substitutions in the nuclear factor EM5 (PIP) motif did not aff ect the pro-B/pre-B specificity of V kappa-J kappa joining. Thus, the PU.1 motif regulates both temporal and tissue-specific rearrangements, while nuclear factor-EMS is not involved in the regulation of Ig kapp a recombination.