Published online 28 April 2005
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Dissecting eukaryotic translation and its control by ribosome density mapping
Howard Hughes Medical Institute, Stanford CA 94305-5428, USA 1Department of Biochemistry, Stanford University Stanford, CA 94305-5307, USA
*To whom correspondence should be addressed. Tel: +1 650 723 9442; Fax: +1 650 723 6783; Email: herschla{at}cmgm.stanford.edu
Received October 26, 2004. Revised February 15, 2005. Accepted March 11, 2005.
Translation of an mRNA is generally divided into three stages: initiation, elongation and termination. The relative rates of these steps determine both the number and position of ribosomes along the mRNA, but traditional velocity sedimentation assays for the translational status of mRNA determine only the number of bound ribosomes. We developed a procedure, termed Ribosome Density Mapping (RDM), that uses site-specific cleavage of polysomal mRNA followed by separation on a sucrose gradient and northern analysis, to determine the number of ribosomes associated with specified portions of a particular mRNA. This procedure allows us to test models for translation and its control, and to examine properties of individual steps of translation in vivo. We tested specific predictions from the current model for translational control of GCN4 expression in yeast and found that ribosomes were differentially associated with the uORFs elements and coding region under different growth conditions, consistent with this model. We also mapped ribosome density along the ORF of several mRNAs, to probe basic kinetic properties of translational steps in yeast. We found no detectable decline in ribosome density between the 5' and 3' ends of the ORFs, suggesting that the average processivity of elongation is very high. Conversely, there was no queue of ribosomes at the termination site, suggesting that termination is not very slow relative to elongation and initiation. Finally, the RDM results suggest that less frequent initiation of translation on mRNAs with longer ORFs is responsible for the inverse correlation between ORF length and ribosomal density that we observed in a global analysis of translation. These results provide new insights into eukaryotic translation in vivo.
Correspondence may also be addressed to Patrick O. Brown. Tel: +1 650 723 0005; Fax: +1 650 725 7811; Email: pbrown{at}cmgm.stanford.edu
Present address: Y. Arava, Department of Biology, Technion, Haifa 32000, Israel
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