Protein kinase C modulates light responses in Neurospora by regulating the blue light photoreceptor WC-1

The Neurospora protein kinase C (NPKC) is a regulator of light responsive genes. We have studied the function of NPKC in light response by investigating its biochemical and functional interaction with the blue light photoreceptor white-collar 1 (WC-1), showing that activation of NPKC leads to a significant decrease in WC-1 protein levels. Furthermore, we show that WC-1 and NPKC interact in a light-regulated manner in vivo, and that protein kinase C (PKC) phosphorylates WC-1 in vitro. We designed dominant negative and constitutively active forms of PKC which are able to induce either a large increase of WC-1 protein level or a strong reduction respectively. Moreover, these changes in PKC activity result in an altered light response. As WC-1 is a key component of Neurospora circadian clock and regulates the clock oscillator component FRQ we investigated the effect of NPKC-mutated forms on FRQ levels. We show that changes in PKC activity affect FRQ levels and the robustness of the circadian clock. Together these data identify NPKC as a novel component of the Neurospora light signal transduction pathway that modulates the circadian clock.     

Circadian-regulated expression of a nuclear-encoded plastid sigma factor gene (sigA) in wheat seedlings.

The activity of a light-responsive psbD promoter in plastids is known to be regulated by a circadian clock. However, the mechanism of the circadian regulation of the psbD light-responsive promotor, which is recognized by an Escherichia coli-type RNA polymerase, is not yet known. We examined the time course of mRNA accumulation of two E. coli-type RNA polymerase subunit genes, sigA and rpoA, under a continuous light condition after 12 h light/12 h dark entrainment. Accumulation of the sigA mRNA was found to be regulated by a circadian clock, while rpoA mRNA did not show any significant oscillation throughout the experiment.     

Two circadian timing circuits in Neurospora crassa cells share components and regulate distinct rhythmic processes

In Neurospora crassa, FRQ, WC-1, and WC-2 proteins comprise the core circadian FRQ-based oscillator that is directly responsive to light and drives daily rhythms in spore development and gene expression. However, physiological and biochemical studies have demonstrated the existence of additional oscillators in the cell that function in the absence of FRQ (collectively termed FRQ-less oscillators [FLOs]). Whether or not these represent temperature-compensated, entrainable circadian oscillators is not known. The authors previously identified an evening-peaking gene, W06H2 (now called clock-controlled gene 16 [ccg-16]), which is expressed with a robust daily rhythm in cells that lack FRQ protein, suggesting that ccg-16 is regulated by a FLO. In this study, the authors provide evidence that the FLO driving ccg-16 rhythmicity is a circadian oscillator. They find that ccg-16 rhythms are generated by a temperature-responsive, temperature-compensated circadian FLO that, similar to the FRQ-based oscillator, requires functional WC-1 and WC-2 proteins for activity. They also find that FRQ is not essential for rhythmic WC-1 protein levels, raising the possibility that this WCFLO is involved in the generation of WC-1 rhythms. The results are consistent with the presence of 2 circadian oscillators within Neurospora cells, which the authors speculate may interact with each other through the shared WC proteins.     

模式生物粗糙脉孢菌光受体的研究进展

粗糙脉孢菌是一种重要的模式生物,在遗传调节机制、昼夜节律运行以及真菌光应答反应研究中起重要的作用.本综述主要介绍粗糙脉孢菌光受体WC-1和VVD的结构与功能,以及它们参与调节昼夜节律和光适应机制方面的研究进展.在该真菌中,所有已知的光应答反应都受蓝光调节,由光受体WC-1和VVD介导.WC-1是该真菌的转录因子,介导最初的光反应过程,产生VVD等多种光反应蛋白,而VVD通过负反馈机制抑制WC-1的转录作用.此外,vvd基因已经用于构建在哺乳动物中表达的光调节基因元件.     

det1, cop1, and cop9 mutations cause inappropriate expression of several gene sets.

Genetic studies using Arabidopsis offer a promising approach to investigate the mechanisms of light signal transduction during seedling development. Several mutants, called det/cop, have been isolated based on their deetiolated/constitutive photomorphogenic phenotypes in the dark. This study examines the specificity of the det/cop mutations with respect to their effects on genes regulated by other signal transduction pathways. Steady state mRNA levels of a number of differently regulated gene sets were compared between mutants and the wild type. We found that det2, cop2, cop3, and cop4 mutants displayed a gene expression pattern similar to that of the wild type. By contrast, det1, cop1, and cop9 mutations exhibited pleiotropic effects. In addition to light-responsive genes, genes normally inducible by plant pathogens, hypoxia, and developmental programs were inappropriately expressed in these mutants. Our data provide evidence that DET1, COP1, and COP9 most likely act as negative regulators of several sets of genes, not just those involved in light-regulated seedling development.     

A two variable delay model for the circadian rhythm of Neurospora crassa

A two variable model with delay in both the variables, is proposed for the circadian oscillations of protein concentrations in the fungal species Neurospora crassa. The dynamical variables chosen are the concentrations of FRQ and WC-1 proteins. Our model is a two variable simplification of the detailed model of Smolen et al. (J. Neurosci. 21 (2001) 6644) modeling circadian oscillations with interlocking positive and negative feedback loops, containing 23 variables. In our model, as in the case of Smolen's model, a sustained limit cycle oscillation takes place in both FRQ and WC-1 protein in continuous darkness, and WC-1 is anti-phase to FRQ protein, as observed in experiments. The model accounts for various characteristic features of circadian rhythms such as entrainment to light dark cycles, phase response curves and robustness to parameter variation and molecular fluctuations. Simulations are carried out to study the effect of periodic forcing of circadian oscillations by light-dark cycles. The periodic forcing resulted in a rich bifurcation diagram that includes quasiperiodicity and chaotic oscillations, depending on the magnitude of the periodic changes in the light controlled parameter. When positive feedback is eliminated, our model reduces to the generic one dimensional delay model of Lema et al. (J. Theor. Biol. 204 (2000) 565), delay model of the circadian pace maker with FRQ protein as the dynamical variable which represses its own production. This one-dimensional model also exhibits all characteristic features of circadian oscillations and gives rise to circadian oscillations which are reasonably robust to parameter variations and molecular noise.