| The two subunits of the circadian RNA-binding protein CHLAMY1 from Chlamydomonas reinhardtii are involved in maintaining period (C1 subunit) and phase (C3 subunit) of the circadian clock (Iliev et al., 2006, Plant Physiol. 142:797-806). Moreover, C1 co-regulates the level of C3. For example, overexpression of C1 causes a parallel increase in C3. Both subunits can also integrate temperature information, resulting in hyper-phosphorylation of C1 and up-regulation of c3 at low temperature (Voytsekh et al., 2008, Plant Physiol. 147:2179-93). Temperature-dependent up-regulation of c3 is mediated predominantly by an E-box element that is situated within the c3 promoter. Notably, the E-box element is also involved in circadian c3 expression as well as in c3 co-regulation by C1 showing that it has multiple roles (Seitz et al., Plant Physiol., in press). Using mobility shift assays and DNA-affinity chromatography along with mass spectrometry, we characterized proteins binding specifically to the E-box region and identified five of them. Their functional characterization is ongoing. Interestingly, we could show that C3 that is also present in the nucleus is part of the E-box binding protein complex. These data indicate a feedback of C3 on its own promoter region. |