Fig. 2 Action of miRNA on a target based on seed sequence complementarity . (a) Perfect seed sequence match leads to fast mRNA degradation. (b) An imperfect match of miRNA with the seed sequence mainly causes translational repression.
These models suggest that the target sites with a match to 2-8 nt of miRNA (8-mer site), 7mer-m8 site, and 7mer site have a high affinity to interact with miRNA. Consequently, they have high miRNA activity.55,56 Reports have shown that the relative contribution and the order of two modes of inhibition (translation repression and mRNA decay) can vary in different organisms.59,60 Some sensitive miRNA studies suggest that majority of repression of target protein production is due to mRNA decay. In zebra fish embryos, the mRNAs with the short poly-A tail are more stable and for such mRNAs, translational repression efficiency is highly correlated with the Poly-A tail length.59 A further kinetics-based study performed in Drosophila S2 and HeLa cells has revealed that gene silencing is achieved initially through translational repression and then followed by mRNA decay.60–62