Comparative utilization of transcription factor GABP by the promoters of ribosomal protein genes rpL30 and rpL32.

The promoters of two mouse ribosomal protein genes, rpL30 and rpL32, contain a similarly located element, called beta, which was previously shown to interact with the same nuclear protein. This protein has now been identified as the GA-binding protein (GABP) on the basis of studies with recombinant GABP subunits and GABP-specific antibodies. The rpL30 element consists of two contiguous GABP binding sites that can form a tetrameric complex with two alpha and two beta 1 subunits of GABP, as well as dimeric complexes with alpha and either the beta 1 or beta 2 subunit. The rpL32 element consists of a solitary GABP binding site that can form only dimeric complexes with alpha and beta 1 or beta 2. Footprint analysis and a comparison of the effects of mutations in each of the tandem rpL30 binding sites demonstrated that the site nearest to the transcriptional start point is strongly favored for dimeric complex formation and is correspondingly more important for rpL30 promoter function. The contributions to overall promoter activity of the proximal rpL30 site and the solitary rpL32 site are virtually the same. Paradoxically, the potential for tetramer formation afforded by the tandem sites in rpL30 has a relatively minor effect on overall promoter strength. These findings illustrate the subtlety of mechanisms by which fine-tuning of rp promoters is achieved.