Transcription factor BACH1 is recruited to the nucleus by its novel alternative spliced isoform

The transcription factor Bach1 is a member of a novel family of broad complex, tramtrack, bric-a-brac/poxvirus and zinc finger (BTB/POZ) basic region leucine zipper factors. Bach1 forms a heterodimer with MafK, a member of the small Maf protein family (MafF, MafG, and MafK), which recognizes the NF-E2/Maf recognition element, a cis-regulatory motif containing a 12-O-tetra-decanoylphorbol-13-acetate-responsive element. Here we describe the gene structure of human BACH1, including a:newly identified promoter and an alternatively RNA-spliced truncated form of BACH1, designated BACH1t, abundantly transcribed in human testis, The alternate splicing originated from the usage of a novel exon located 5.6 kilobase pairs downstream of the exon encoding, the leucine zipper domain, and produced a protein that contained the conserved BTB/POZ, Cap'n collar, and basic region domains, but lacked the leucine zipper domain essential for NF-E2/Maf recognition element binding, Subcellular localization studies using green fluorescent protein as a reporter showed that full-length BACH1: localized to the cytoplasm, whereas BACH1t accumulated in the nucleus. Interestingly, coexpression of BACH1 and BACH1t demonstrated interaction between the molecules and the induction of nuclear: import of BACH1. These results suggested that BACH1t recruits BACH1 to the nucleus through BTB domain-mediated interaction.