Alu elements are the most abundant repetitive elements in the human genome; they 
emerged from the signal recognition particle RNA gene and are composed of two 
related but distinct monomers (left and right arms). Alu RNAs transcribed from 
these elements are present at low levels at normal cell growth but various 
stress conditions increase their abundance. Alu RNAs are known to bind the 
cognate proteins SRP9/14. We purified synthetic Alu RNP, composed of Alu RNA in 
complex with SRP9/14, and investigated the effects of Alu RNPs and naked Alu RNA 
on protein translation. We found that the dimeric Alu RNP and the monomeric left 
and right Alu RNPs have a general dose-dependent inhibitory effect on protein 
translation. In the absence of SRP9/14, Alu RNA has a stimulatory effect on all 
reporter mRNAs. The unstable structure of sRight RNA suggests that the 
differential activities of Alu RNP and Alu RNA may be explained by 
conformational changes in the RNA. We demonstrate that Alu RNPs and Alu RNAs do 
not stably associate with ribosomes during translation and, based on the 
analysis of polysome profiles and synchronized translation, we show that Alu RNP 
and Alu RNA regulate translation at the level of initiation.