ZBP-99 Defines a Conserved Family of Transcription Factors and Regulates Ornithine Decarboxylase Gene Expression

Abstract

Among transcription factors that regulate ornithine decarboxylase (ODC) gene expression are those that interact with GC-rich promoters, including Sp1 and ZBP-89. Sp1 functions as a transactivator and ZBP-89 as a transrepressor of both the ODC and gastrin promoters. This study reports the cloning and characterization of a second member of the ZBP family that also binds GC boxes. ZBP-99 contains four Krüppel-type zinc fingers that collectively share 91% amino acid sequence similarity and 79% sequence identity with those found in ZBP-89. In addition, there are highly conserved amino acid sequences in the carboxy-terminal segments of the two genes. In spite of their structural similarities, the two proteins are encoded at distinct loci, ZBP-89 on chromosome 3q21 and ZBP-99 on 1q32.1. The predicted open reading frame of ZBP-99 cDNA encodes a 99-kDa protein. Electrophoretic mobility shift assays showed that ZBP-99 protein specifically binds to the GC-rich promoter elements of gastrin and ODC genes. Northern blot analysis showed that a major ZBP-99 transcript of 5.6 kb is expressed ubiquitously at low levels, with elevated expression levels in placenta and in adult kidney, liver, and lymphocytes. Cotransfection of AGS gastric adenocarcinoma and HT-29 colon adenocarcinoma cells with a ZBP-99 expression construct and with an ODC reporter construct show that ZBP-99 repressed basal expression in the two cell lines by 80 and 60%, respectively. Collectively, the data suggest that ZBP-99 binds GC-rich promoters and may complement the activities mediated by ZBP-89.