BACKGROUND: MicroRNA (miRNA) are diverse in sequence and have a single known sequence bias: they tend to start with uridine (U).
RESULTS: Our analyses of fly, worm and mouse miRNA sequence data reveal that the 5\u27-U is recognized after miRNA production. Only one of the two strands can be assembled into Argonaute protein from a single miRNA/miRNA* molecule: in fly embryo lysate, a 5\u27-U promotes miRNA loading while decreasing the loading of the miRNA*.
CONCLUSION: We suggest that recognition of the 5\u27-U enhances Argonaute loading by a mechanism distinct from its contribution to weakening base pairing at the 5\u27-end of the prospective miRNA and, as recently proposed in Arabidopsis and in humans, that it improves miRNA precision by excluding incorrectly processed molecules bearing other 5\u27-nt
