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Functional Characteristics of Gene Expression Motifs with Single and Dual Strategies of Regulation

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.