Structure-function analysis of TFII-I. Roles of the N-terminal end, basic region, and I-repeats.

The transcription factor TFII-I can bind specifically to several DNA sequence elements and is implicated in both basal and activated transcription. There are four alternatively spliced isoforms of TFII-I, all characterized by the presence of six I-repeats, R1-R6, each containing a potential helix-loop-helix motif implicated in protein-protein interactions. These isoforms exhibit both homomeric and heteromeric interactions that lead to nuclear localization. In this study we mapped two distinct regions in TFII-I that affect its DNA binding. Deletion of either of these regions led to abrogation of DNA binding and transcriptional activation from both the Vbeta and c-fos promoters. The I-repeats, as expected, were capable of mediating homomeric interactions either individually or in combination. Unexpectedly, an additional homomeric interaction domain was found within the N-terminal end of TFII-I that includes a putative leucine zipper motif. These data suggest a model in which TFII-I undergoes regulated homomeric interaction mediated by both the N-terminal end and the I-repeats.