We add our observation that miRNAmediated deadenylation is evident prior to the onset of measurable translational repression. Taken together, it is thus apparent across different systems that Danshensu miRNAs can stimulate deadenylation without a specific requirement for mRNA translation or a prior translational repression step. Atropine sulfate Alternatively, deadenylation may exclusively stimulate mRNA decay. This has been addressed in mammalian and D. melanogaster cells using reporter constructs designed to give rise to mRNAs either carrying the 39 end stem-loop of histone mRNA instead of a tail, or lacking a tail due to the presence of a self-cleaving ribozyme in the 39 UTR. These mRNAs could still be silenced, although at times not as strongly as their polyadenylated counterparts. The common observation is thus that miRNAs can repress translation without a requirement for mRNA deadenylation. It does, however, not necessarily follow that deadenylation has no role in translational repression, as shown by our present findings and previous reports that translational repression is enhanced by the presence of a poly tail. Most importantly, we show here that while blocking let-7-mediated deadenylation does not completely abolish let-7-mediated translational repression, it nevertheless clearly attenuated it. This is a direct demonstration in living cells that mRNA deadenylation can specifically contribute to translational repression by a miRNA. Taken together with the extant literature, our data thus favours a model whereby mRNA deadenylation is a proximal miRNA action that not only promotes mRNA decay but also attenuates the stimulatory role of the poly tail in mRNA translation, by generating an oligo-adenylated form of the mRNA that associates less with cytoplasmic PABP. Indeed, mammalian miRNA target mRNAs are enriched in Ago and GW182 immunoprecipitations, while they are underrepresented in complexes containing PABPC4. The oligo-adenylated mRNA is further silenced by corepressors, which most plausibly interfere with cap/eIF4E function, and then is either stably stored or destroyed by decapping and exonucleolytic degradation. The magnitude of translational repression due to miRNA-mediated deadenylation may depend on the extent to which the poly tail contributed to target mRNA translation before removal; this will vary between mRNAs and with the functional state of the cellular translational machinery. For instance, cells may express variable amounts of several variants of cytoplasmic PABP and PABP activity is know to be regulated by specific inhibitor proteins such as PAIP2. Such differences may explain some of the divergent literature on miRNA-responses to the absence of a poly tail on target mRNA. The balance between decay and storage of target mRNA may further be determined through interplay between the specific sequence context and the given cellular environment. With these provisos, the model explains most reports assessing the role of the poly tail in miRNA-mediated repression. The above scenario clearly does not accommodate postinitiation effects by miRNAs, and several reports have further presented experimental evidence or theoretical arguments to link miRNA-mediated repression to late sub-steps of translation initiation. More work will have to be done to address the possibility that miRNAs affect translation at more than one step. The poly tail also stimulates the 60S subunit joining step during late initiation and it is linked to translation termination through an interaction between PABP and eRF3, providing ways to rationalise a role for deadenylation in several alternative models of translational repression by miRNAs.