The role of an E-box element: multiple frunctions and interacting partners

Abstract

Circadian clocks can be entrained by light-dark or temperature cycles. In the green alga Chlamydomonas reinhardtii, 12h changes in temperature between 18°C and 28°C synchronize its clock. Both subunits of the circadian RNA-binding protein CHLAMY1, named C1 and C3, are able to integrate temperature information. C1 gets hyper-phosphorylated in cells grown at 18°C and the level of C3 is up-regulated at this temperature. In the long period mutant per1, where temperature entrainment is disturbed, the temperature-dependent regulation of C1 and C3 is altered. Up-regulation of C3 at the low temperature is mediated predominantly by an E-box element situated in its promoter region. This cis-acting element is also relevant for circadian expression of c3 as well as of its up-regulation in cells, where C1 is overexpressed. Among the few identified factors interacting with the E-box region, C3 is also present, suggesting that it feedbacks on its own transcription.

Figure 1

Multiple roles of the E-box region. The C1 and C3 subunits of the circadian RNA-binding protein CHLAMY1 are shown and their distribution in the cytoplasm and nucleus is indicated. All factors that bind to the E-box region of the c3 promoter are depicted as well as the conditions for increasing c3 gene expression via the E-box region. These are low temperature, night phase within a circadian cycle and the co-regulation of c3 upon overexpression of C1. Since C1 was not found in nuclear extracts, a factor X is postulated that is involved in upregulation of c3 within the co-regulation mechanism. Additionally, the CHLAMY1 complex formation by interaction of C1 and C3 in the cytoplasm is shown. The CHLAMY1 complex binds to UG ≥7 repeat sequences of mRNAs in a circadian manner and thus influences their expression (reviewed in ref. 5).