Fig. 1.

Translational control and C/EBPβ isoform regulation. Growth factor, hormone, nutrition, and stress signals are integrated by the mammalian mTOR kinase that monitors the growth and differentiation status of a cell. The mTOR kinase (low activity (blue); high activity (red)) regulates the function of the translation initiation machinery. The cis-regulatory upstream open reading frame uORF in the C/EBPβ mRNA senses the activity of the translation machinery and directs initiation, leaky scanning, and reinitiation. High activity of the translation machinery directs reinitiation and production of the truncated “repressor” isoform LIP of C/EBPβ. Lowering the activity of the initiation machinery results in leaky scanning over the uORF ATG and translation initiation at the first ATG, resulting in the production of the long “activator” isoform LAP. Rapamycin inhibits mTOR signaling, resulting in the production of mainly the LAP isoform