Transcriptional activation by LR1 at the Eμ enhancer and switch region sites

LR1 is a B cell-specific, sequence-specific duplex DNA binding activity which is induced in B cells carrying out class switch recombination, Here we identify several properties of LR1 which enable it to function in transcriptional regulation. We show that LR1 contributes to transcriptional activation by the E mu immunoglobulin heavy chain intron enhancer by binding to a site within the enhancer core. We further show that LR1 bends DNA upon binding. In addition, we show that LR1 is itself a bona fide transcriptional activator, as multimerized LR1 sites produce an element which can enhance transcription from a minimal promoter. In order for class switch recombination to occur, an activating signal must be transmitted via the E mu core, and both S regions targeted for recombination must be actively transcribed. The properties of LR1 that we have identified suggest distinct potential functions of LR1 duplex DNA binding activity in class switch recombination. First, LR1 may contribute to recombinational activation by the E mu core. Second, there are multiple potential LR1 duplex binding sites in each of the G-rich switch regions, and LR1 bound at contiguous sites may enhance recombination by stimulating transcription of the S regions.