I've been doing a lot of research on Alu repeats and how they mediate the gene expression. I read the following article "Useful junk: Alu RNAs in the human transcriptome".
And it says that alu repeats embedded in 5'UTR makes stable secondary structure that prevents the assembly of ribosomal subunits, thus blocking the process of translation. I've been reading various articles to find out why Alu repeats form this secondary structure and why only in the 5'UTR and not in the 3'UTR. But haven't been able to find an answer.