Using altered specificity Oct‐1 and Oct‐2 mutants to analyze the regulation of immunoglobulin gene transcription

Oct‐1 and Oct‐2 represent the prototypical example of related transcription factors with identical DNA recognition properties. They bind functionally critical octamer elements found in diverse regulatory sequences. It has not been possible to determine directly if these factors are functionally redundant or selective when interacting with different regulatory sequences in the appropriate cell type. An equivalent pair of altered DNA‐binding specificity mutants of Oct‐1 and Oct‐2 are used to examine their function from varied regulatory contexts in B cells. These factors function as redundant activators of immunoglobulin (Ig) gene promoters (Vκ and V H ) and a histone H2B promoter. The structural basis of redundancy resides in the highly conserved DNA‐binding POU domain, because this domain of either protein can activate transcription from both Ig and H2B promoters. We find that the nature of a distal enhancer dictates the relative potency of Oct‐1 versus Oct‐2 bound to a promoter. Oct‐1 preferentially stimulates transcription from a V H or Vκ promoter in combination with enhancers from the IgH or Igκ locus, respectively. In this context, the more potent action of Oct‐1 is dependent on a region external to the POU domain. These results suggest that Oct‐1 may be the critical regulator of Ig gene transcription during B cell development and provide an explanation for selective Ig isotype expression defects in Oct‐2 and OCA‐B null mice.