Animal microRNAs (miRNAs) regulate gene expression by inhibiting translation and/or by inducing degradation of target messenger RNAs. It is unknown how much translational control is exerted by miRNAs on a genome-wide scale. We used a new proteomic approach to measure changes in synthesis of several thousand proteins in response to miRNA transfection or endogenous miRNA knockdown. In parallel, we quantified mRNA levels using microarrays. Here we show that a single miRNA can repress the production of hundreds of proteins, but that this repression is typically relatively mild. A number of known features of the miRNA-binding site such as the seed sequence also govern repression of human protein synthesis, and we report additional target sequence characteristics. We demonstrate that, in addition to downregulating mRNA levels, miRNAs also directly repress translation of hundreds of genes. Finally, our data suggest that a miRNA can, by direct or indirect effects, tune protein synthesis from thousands of genes.

Spatiotemporal control of gene expression is crucial for development and subject to evolutionary changes. Although proteins are the final product of most genes, the developmental proteome of an animal has not yet been comprehensively defined, and the correlation between mRNA and protein abundance during development is largely unknown. Here, we globally measured and compared protein and mRNA expression changes during the life cycle of the nematodes C. elegans and C. briggsae, separated by ~30 million years of evolution. We observed that developmental mRNA and protein changes were highly conserved to a surprisingly similar degree but were poorly correlated within a species, suggesting important and widespread posttranscriptional regulation. Posttranscriptional control was particularly well conserved if mRNA fold changes were buffered on the protein level, indicating a predominant repressive function. Finally, among divergently expressed genes, we identified insulin signaling, a pathway involved in lifespan determination, as a putative target of adaptive evolution.

Regulated degradation of circadian clock proteins is a crucial step for rhythm generation per se but also for establishing a normal circadian period. Here, the authors show that the F-box protein beta-transducin repeat containing protein 1 (beta-TrCP1) as part of the E3 ubiquitin ligase complex is an essential component of the mammalian circadian oscillator. Down-regulation of endogenous beta-TrCP1 as well as expression of a dominant-negative form both result in lengthening of the circadian period in oscillating fibroblasts. These phenotypes are due to an impaired degradation of PERIOD (PER) proteins, since expression of beta-TrCP interaction-deficient PER2 variants--but not wild-type PER2--results in a dramatic stabilization of PER2 protein as well as in the disruption of circadian rhythmicity. Mathematical modeling conceptualizes the authors' findings and suggests that loss of sustained rhythmicity in cells with eliminated beta-TrCP-mediated PER2 degradation is due to excessive nuclear repression, a prediction they verified experimentally.