Intracellular delay limits cyclic changes in gene expression
  Based on previously published experimental observations and mathematical models for Hes1, p53 and NF-kB gene expression, we improve these models through a distributed delay formulation of the time lag between transcription factor binding and mRNA production. This description of natural variability for delays introduces a transition from a stable steady state to limit cycle oscillations and then a second transition back to a stable steady state which has not been observed in previously published models. On the basis of our results and following recent discussions about the role of delay-induced oscillations in gene transcription we establish the hypothesis that the period of the delay-induced cyclic changes should be characterized by an upper bound so that it cannot be greater then the period of fundamental biological cycles. We demonstrate our approach for two models. The first model describes Hes1 autorepression with equations for Hes1 mRNA production and Hes1 protein translation. The second model describes Hes1 repression by the protein complex Gro/TLE1/Hes1, where Gro/TLE1 is activated by Hes1 phosphorylation. 
Katja Rateitschak, Olaf Wolkenhauer