LXRalpha is an orphan member of the nuclear hormone receptor superfamily that displays constitutive transcriptional activity. We reasoned that this activity may result from the production of an endogenous activator that is a component of intermediary metabolism. The use of metabolic inhibitors revealed that mevalonic acid biosynthesis is required for LXRalpha activity. Mevalonic acid is a common metabolite used by virtually all eukaryotic cells. It serves as a precursor to a large number of important molecules including farnesyl pyrophosphate, geranylgeranyl pyrophosphate, cholesterol, and oxysterols. Inhibition of LXRalpha could be reversed by addition of mevalonic acid and certain oxysterols but not by other products of mevalonic acid metabolism. Surprisingly, the constitutive activity of LXRalpha was inhibited by geranylgeraniol, a metabolite of mevalonic acid. These findings suggest that LXRalpha may represent a central component of a signaling pathway that is both positively and negatively regulated by multiple products of mevalonate metabolism.

A high level of accuracy during protein synthesis is considered essential for life. Aminoacyl-tRNA synthetases (aaRSs) translate the genetic code by ensuring the correct pairing of amino acids with their cognate tRNAs. Because some aaRSs also produce misacylated aminoacyl-tRNA (aa-tRNA) in vivo, we addressed the question of protein quality within the context of missense suppression by Cys-tRNA Pro , Ser-tRNA Thr , Glu-tRNA Gln , and Asp-tRNA Asn . Suppression of an active-site missense mutation leads to a mixture of inactive mutant protein (from translation with correctly acylated aa-tRNA) and active enzyme indistinguishable from the wild-type protein (from translation with misacylated aa-tRNA). Here, we provide genetic and biochemical evidence that under selective pressure, Escherichia coli not only tolerates the presence of misacylated aa-tRNA, but can even require it for growth. Furthermore, by using mass spectrometry of a reporter protein not subject to selection, we show that E. coli can survive the ambiguous genetic code imposed by misacylated aa-tRNA tolerating up to 10% of mismade protein. The editing function of aaRSs to hydrolyze misacylated aa-tRNA is not essential for survival, and the EF-Tu barrier against misacylated aa-tRNA is not absolute. Rather, E. coli copes with mistranslation by triggering the heat shock response that stimulates nonoptimized polypeptides to achieve a native conformation or to be degraded. In this way, E. coli ensures the presence of sufficient functional protein albeit at a considerable energetic cost.accuracy ͉ aminoacyl-tRNA ͉ fidelity ͉ protein synthesis ͉ missense suppression

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