Abundant conserved microRNA target sites in the 5'-untranslated region and coding sequence.

Recent studies have shown that miRNAs can target the promoter and CDS region. Thus, we predicted miRNA target sites in the 5'-UTR, CDS and 3'-UTR of Homo sapiens, Mus musculus and Drosophila melanogaster using miRanda and TargetScan. Target-site densities normalized with the average region length were higher in the 5'-UTR than 3'-UTR in all three organisms but were lower in the negative data set. Interestingly, the putative target sites were more conserved than non-target regions in both the 5'-UTR and 3'-UTR, implying that target sites in the 5'-UTR are subject to high selective pressure and might be functional. In Drosophila, 48 of 78 (61.5%) miRNAs showed high similarities with predicted siRNAs. Based on the results of previous experimental studies and a large-scale statistical analysis, we conclude that miRNA-mediated regulation is not limited to the 3'-UTR. However, the functionality of target sites in the 5'-UTR and CDS requires thorough investigation.