  MicroRNAs are small noncoding RNAs that regulate genes post-transciptionally
by binding and degrading target eukaryotic mRNAs. We use a quantitative model
to study gene regulation by inhibitory microRNAs and compare it to gene
regulation by prokaryotic small non-coding RNAs (sRNAs). Our model uses a
combination of analytic techniques as well as computational simulations to
calculate the mean-expression and noise profiles of genes regulated by both
microRNAs and sRNAs. We find that despite very different molecular machinery
and modes of action (catalytic vs stoichiometric), the mean expression levels
and noise profiles of microRNA-regulated genes are almost identical to genes
regulated by prokaryotic sRNAs. This behavior is extremely robust and persists
across a wide range of biologically relevant parameters. We extend our model to
study crosstalk between multiple mRNAs that are regulated by a single microRNA
and show that noise is a sensitive measure of microRNA-mediated interaction
between mRNAs. We conclude by discussing possible experimental strategies for
uncovering the microRNA-mRNA interactions and testing the competing endogenous
RNA (ceRNA) hypothesis.
