  MicroRNAs can affect the protein translation using nine mechanistically
different mechanisms, including repression of initiation and degradation of the
transcript. There is a hot debate in the current literature about which
mechanism and in which situations has a dominant role in living cells. The
worst, same experimental systems dealing with the same pairs of mRNA and miRNA
can provide ambiguous evidences about which is the actual mechanism of
translation repression observed in the experiment. We start with reviewing the
current knowledge of various mechanisms of miRNA action and suggest that
mathematical modeling can help resolving some of the controversial
interpretations. We describe three simple mathematical models of miRNA
translation that can be used as tools in interpreting the experimental data on
the dynamics of protein synthesis. The most complex model developed by us
includes all known mechanisms of miRNA action. It allowed us to study possible
dynamical patterns corresponding to different miRNA-mediated mechanisms of
translation repression and to suggest concrete recipes on determining the
dominant mechanism of miRNA action in the form of kinetic signatures. Using
computational experiments and systematizing existing evidences from the
literature, we justify a hypothesis about co-existence of distinct
miRNA-mediated mechanisms of translation repression. The actually observed
mechanism will be that acting on or changing the limiting "place" of the
translation process. The limiting place can vary from one experimental setting
to another. This model explains the majority of existing controversies
reported.
