Complex Interactions at a GC-rich Domain Regulate Cell Type-dependent Activity
of the Ornithine Decarboxylase Promoter
Run-Sheng Li, Mitchell S. Abrahamsen, Ronda R Johnson, and David R. Morris
Regulation of ornithine decarboxylase (ODC) is critical to the control
of cellular growth, differentiation, and carcinogenesis. A GC-rich region
in the ODC promoter contains two overlapping protein binding sites that
interact to regulate basal level expression in some cell types. A perfect
binding motif for transcription factor Sp1 (CCCCGCCCC) is located at nucleotides
-114 to -106 relative to the site of transcriptional initiation, binds
strongly to purified Sp1 protein, and forms several complexes when incubated
with nuclear extracts. Only one of these complexes is recognized by Sp1-specific
antibody. A new protein-binding motif (GCCCCTCCCCC, located at -110 to
-100) partially overlaps with the Spl site and analysis by DNase I protection
showed that a new protein (NF-ODC1) and the Spl-like proteins interact
with the ODC promoter in a mutually exclusive manner. Mutation of the NF-ODC1
binding motif strongly enhanced ODC promoter strength in some cell types,
but had little or no influence in others. The effect of mutating the Sp1
site also varied with cell type. These cell type specificities did not
correlate with the levels of Sp1 and NF-ODC1 binding activities in nuclear
extracts. These results show that regulation of the ODC promoter by the
Sp1 family is cell type-specific and modulated by a negative effector that
we have termed NF-ODC1.