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