Exemplary loci illustrating precision and variability in miRNA processing. (A) Most miRNAs, such as mir-184, exhibit precisely defined 5′ ends of both miRNA and star species. Since the mature strand of mir-184 is highly biased over its star species, there is one dominant miRNA-type regulatory species produced from this locus. (B) mir-193 is a locus exhibiting balanced accumulation of small RNAs from its hairpin arms. In addition, both 5p and 3p arms exhibit abundant secondary and even tertiary 5′ isomiR species. All of these accumulate in AGO1; therefore, mir-193 produces at least five substantial miRNA-type regulatory RNAs. Note that the 3p RNAs also accumulate in AGO2 as evidenced by their enrichment in a library prepared from small RNAs resistant to oxidization.
