  Transcriptional regulation by transcription factors and post-transcriptional
regulation by microRNAs constitute two major modes of regulation of gene
expression. While gene expression motifs incorporating solely transcriptional
regulation are well investigated, the dynamics of motifs with dual strategies
of regulation, i.e., both transcriptional and post-transcriptional regulation,
have not been studied as extensively. In this paper, we probe the dynamics of a
four-gene motif with dual strategies of regulation of gene expression. Some of
the functional characteristics are compared with those of a two-gene motif, the
genetic toggle, employing only transcriptional regulation. Both the motifs
define positive feedback loops with the potential for bistability and
hysteresis. The four-gene motif, contrary to the genetic toggle, is found to
exhibit bistability even in the absence of cooperativity in the regulation of
gene expression. The four-gene motif further exhibits a novel dynamical feature
in which two regions of monostability with linear threshold response are
separated by a region of bistability with digital response.
  Using the linear noise approximation (LNA), we further show that the
coefficient of variation (a measure of noise), associated with the protein
levels in the steady state, has a lower magnitude in the case of the four-gene
motif as compared to the case of the genetic toggle. We next compare
transcriptional with post-transcriptional regulation from an information
theoretic perspective. We focus on two gene expression motifs, Motif 1 with
transcriptional regulation and Motif 2 with post-transcriptional regulation.
  We show that amongst the two motifs, Motif 2 has a greater capacity for
information transmission for an extended range of parameter values.
