Regulation of Translational Efficiency by Disparate5′-UTRs of PPARγSplice Variants

The PPAR- γ gene encodes for at least 7 unique transcripts due to alternative splicing of five exons in the 5′-untranslated region (UTR). The translated region is encoded by exons 1–6, which are identical in all isoforms. This study investigated the role of the 5′-UTR in regulating the efficiency with which the message is translated to protein. A coupled in vitro transcription-translation assay demonstrated that PPAR- γ 1, - γ 2, and - γ 5 are efficiently translated, whereas PPAR- γ 4 and - γ 7 are poorly translated. An in vivo reporter gene assay using each 5′-UTR upstream of the firefly luciferase gene showed that the 5′-UTRs for PPAR- γ 1, - γ 2, and - γ 4 enhanced translation, whereas the 5′-UTRs for PPAR- γ 5 and - γ 7 inhibited translation. Models of RNA secondary structure, obtained by the mfold software, were used to explain the mechanism of regulation by each 5′-UTR. In general, it was found that the translational efficiency was inversely correlated with the stability of the mRNA secondary structure, the presence of base-pairing in the consensus Kozak sequence, the number of start codons in the 5′-UTR, and the length of the 5′-UTR. A better understanding of posttranscriptional regulation of translation will allow modulation of protein levels without altering transcription.