  According to the `ceRNA hypothesis', microRNAs (miRNAs) may act as mediators
of an effective positive interaction between long coding or non-coding RNA
molecules, carrying significant potential implications for a variety of
biological processes. Here, inspired by recent work providing a quantitative
description of small regulatory elements as information-conveying channels, we
characterize the effectiveness of miRNA-mediated regulation in terms of the
optimal information flow achievable between modulator (transcription factors)
and target nodes (long RNAs). Our findings show that, while a sufficiently
large degree of target derepression is needed to activate miRNA-mediated
transmission, (a) in case of differential mechanisms of complex processing
and/or transcriptional capabilities, regulation by a post-transcriptional
miRNA-channel can outperform that achieved through direct transcriptional
control; moreover, (b) in the presence of large populations of weakly
interacting miRNA molecules the extra noise coming from titration disappears,
allowing the miRNA-channel to process information as effectively as the direct
channel. These observations establish the limits of miRNA-mediated
post-transcriptional cross-talk and suggest that, besides providing a degree of
noise buffering, this type of control may be effectively employed in cells both
as a failsafe mechanism and as a preferential fine tuner of gene expression,
pointing to the specific situations in which each of these functionalities is
maximized.
