The B cell coactivator Bob1 shows DNA sequence-dependent complex formation with Oct-1/Oct-2 factors, leading to differential promoter activation.

Abstract

We have shown previously that both octamer binding transcription factors, namely the ubiquitous Oct‐1 and the B cell‐specific Oct‐2A protein, can be enhanced in transcriptional activity by their association with the B cell‐specific coactivator protein Bob1, also called OBF‐1 or OCA‐B. Here we study the structural requirements for ternary complex formation of DNA‐Oct‐Bob1 and coactivation function of Bob1. In analogy to DNA‐bound transcription factors, Bob1 has a modular structure that includes an interaction domain (amino acids 1–65) and a C‐terminal domain (amino acids 65–256), both important for transcriptional activation. A mutational analysis has resolved a region of seven amino acids (amino acids 26–32) in the N‐terminus of Bob1 that are important for contacting the DNA binding POU domain of Oct‐1 or Oct‐2. In contrast to the viral coactivator VP16 (vmw65), which interacts with Oct‐1 via the POU homeosubdomain, Bob1 association with Oct factors requires residues located in the POU‐specific subdomain. Because the same residues are also involved in DNA recognition, we surmised that this association would affect the DNA binding specificity of the Oct‐Bob1 complex compared with free Oct factors. While Oct‐1 or Oct‐2 bind to a large variety of octamer sequences, Bob1 ternary complex formation is indeed highly selective and occurs only in a subset of these sequences, leading to the differential coactivation of octamer‐containing promoters. The results uncover a new level in selectivity that furthers our understanding in the regulation of cell type‐specific gene expression.