Oct-1 POU and octamer DNA co-operate to recognise the Bob-1 transcription co-activator via induced folding

The expression of immunoglobulin genes is controlled in part by the DNA-bin ding protein Oct-1 and the B cell-specific transcription co-activator, Bob1 (also known as OCA-B or OBF-1) that together form a complex on the Ig kapp a promoter. We have characterised the assembly of the ternary complex using biophysical methods. Bob1 binds specifically as a monomer to the complex o f the Oct-1 DNA-binding domain (Oct-1 POU) and the Ig kappa promoter, but b inds weakly to either Oct-1 POU or the Ig kappa promoter alone, indicating that both are required to make an avid complex. Ternary complex formation r equires a defined DNA sequence, as the stability of the complex can be stro ngly affected by a single basepair change or by removing 5-methyl groups fr om selected thymine bases. In isolation, Bob1 appears to have little secondary structure, but may beco me partially structured upon recruitment into the ternary complex as demons trated by circular dichroism spectra and calorimetry. These and other findi ngs suggest that ternary complex formation requires a defined geometry of t he POU/DNA complex, and that the co-activator makes stereo-specific contact s to both the POU protein and the major groove of the DNA that induces its fold. (C) 1999 Academic Press.