Close, But Not Too Close

MicroRNAs (miRNAs) in plants are generally highly complementary to their target RNAs, yet, in most animal miRNAs, only the 8-nucleotide "seeds" sequence bases pair fully with the target, with few base pairs between the remainder of the miRNA and target. Plant miRNAs are methylated at their 3' ends, whereas animals' miRNAs are not. Ameres et al. (p. 1534; see the Perspective by Pasquinelli) noticed that, in fruit flies, miRNAs engineered to have high complementarity to target RNAs were present at reduced levels. These miRNAs were trimmed and uridylated at their 3' ends, features involved in RNA degradation. Fly small interfering RNAs, all of which are methylated at their 3' ends, were unaffected, unless the methylating enzyme, Hen1, was mutated. Thus, 3'-methylation may prevent complementarity-driven remodeling and degradation of small RNAs.