Genomic organization of the mouse peroxisome proliferator-activated receptor β/δ gene: alternative promoter usage and splicing yield transcripts exhibiting differential translational efficiency

Peroxisome proliferator-activated receptor (PPAR) β/Δ is ubiquitously expressed, but the level of expression differs markedly between different cell types. In order to determine the molecular mechanisms governing PPARβ/Δ gene expression, we have isolated and characterized the mouse gene encoding PPARβ/Δ. The gene spans approx. 41 kb and comprises 11 exons of which the six exons located in the 3′-end of the gene are included in all transcripts. Primer-extension and 5′-rapid amplification of cDNA ends experiments revealed the presence of multiple transcription start points and splice variants, originating from the use of at least four different promoters. One of these transcription start points was found to be used predominantly in all tissues examined. Initiation from this major transcription start point gives rise to a transcript with a 548nt 5′-untranslated leader containing eight upstream AUG codons. We show that the presence of the 548nt leader resulted in a low translational efficiency of the corresponding PPARβ/Δ mRNA and propose, based on structural features of the 5′-untranslated region, that translational initiation may be mediated via an internal ribosome entry site-dependent mechanism.