In Arabidopsis thaliana, many circadian clock-associated genes have been identified. Among them, the evening-expressed TOC1 (TIMING OF CAB EXPRESSION 1) gene plays a role by forming a transcriptional feedback core loop together with the morning-expressed CCA1 (CIRCADIAN CLOCK-ASSOCIATED 1) gene and its homologous LHY (LATE ELONGATED HYPOCOTYL) gene. TOC1 encodes a member of the PSEUDO-RESPONSE REGULATOR (PRR) family, including PRR9, PRR7, PRR5, PRR3,and PRR1/TOC1. The PRR genes other than TOC1 (or PRR1) also appear to be crucial for certain circadian-associated events. To clarify missing genetic linkages amongst these PRR genes, here we constructed a toc1 prr5 double knockdown mutant. In free-running circadian rhythms, the resulting toc1-2 prr5-11 mutant plants showed an extremely short period and reduced amplitude phenotype, which was more severe than that of the toc1-2 single mutant plant, suggesting a non-linear genetic interaction between TOC1 and PRR5. Surprisingly, the hallmark early flowering phenotype of toc1-2 in the short-day conditions had been converted to a markedly late flowering phenotype in the long-day conditions, when combined with the prr5-11 allele, which itself showed a subtle flowering phenotype. This unexpected genetic result (i.e. phenotypic sign conversion) suggested that the TOC1 and PRR5 genes are coordinately implicated in a non-linear and closed genetic circuitry. In the toc1-2 prr5-11 double mutant, the diurnal expression profile of CDF1 (CYCLING DOF FACTOR 1) was markedly de-repressed in the evening in the long-day conditions. These and other results of this study led us to propose the novel view that TOC1 might play bipartite roles in the control of flowering time within a closed circuitry; the one is a GI (GIGANTEA)-dependent negative role through CCA1/LHY, and the other is a CDF1-dependent positive role through cooperating closely with PRR5.
Schizosaccharomyces pombe and Saccharomyces cerevisiae are excellent model organisms to study lifespan. We conducted screening to identify novel genes that, when overexpressed, extended the chronological lifespan of fission yeast. We identified seven genes, among which we focused on SPBC16A3.08c. The gene product showed similarity to Ylr150w of S. cerevisiae, which has affinity for guanine-quadruplex nucleic acids (G4). The SPBC16A3.08c product associated with G4 in vitro and complemented the phenotype of an S. cerevisiae Ylr150w deletion mutant. From these results, we proposed that SPBC16A3.08c encoded for a functional homolog of Ylr150w, which we designated ortholog of G4-associated protein (oga1 (+)). oga1 (+) overexpression extended the chronological lifespan and also decreased mating efficiency and caused both high and low temperature-sensitive growth. Deleting oga1 (+) resulted in caffeine-sensitive and canavanine-resistant phenotypes. Based on these results, we discuss the function of Oga1 on the chronological lifespan of fission yeast.
In Arabidopsis thaliana, a consistent multiloop clock model has been widely adopted in many recent publications. This tentative model consists of three interactive feedback loops, namely the core CCA1/LHY-TOC1/X loop, the morning CCA1/LHY-PRR9/PRR7 loop and the evening Y-TOC1 loop, in which the undefined Y gene might be GI. The model in its current form provides us with a basis on which to address a number of fundamental issues for a better understanding of the molecular mechanism by which the central oscillator generates circadian rhythms. We have been conducting a series of genetic studies through the establishment of a set of combinatorial mutants. We have already characterized a prr9 prr7 double loss-of-function mutant that has lost the morning loop, and a cca1 lhy toc1 triple mutant that lacks the core loop. Extension of this line of study required characterization of a gi toc1 double loss-of-function mutant, which is expected to have no evening loop, and a prr9 prr7 toc1 triple mutant, lacking both the morning and evening loops. Genetic analysis of both these lines is reported here. From the results, we have clarified the genetic linkages between GI and TOC1 and those between PRR9/PRR7 and TOC1 with reference to the circadian clock-associated phenotypes, including: (i) length of hypocotyls during early photomor-phogenesis; (ii) photoperiodic control of flowering time; and (iii) expression profiles of CCA1 and LHY under free-running conditions. These results indicate that GI is not sufficient to fulfill the Y role, but plays more complicated clock-associated roles and, interestingly, that no epistatic interaction between PRR9/PRR7 and TOC1 was observed. Furthermore, these clock-defective mutants could still generate robust, free-running rhythms at the level of transcription. Therefore, we speculate that an as yet undefined oscillator (or loop) continues to generate rhythms within the plants lacking GI/TOC1 or PRR9/PRR7/TOC1.
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