期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2015
卷号：112
期号：15
页码：E1916-E1925
DOI：10.1073/pnas.1504576112
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceCyanobacteria are increasingly being considered for use in large-scale outdoor production of fuels and industrial chemicals. Cyanobacteria can anticipate daily changes in light availability using an internal circadian clock and rapidly alter their metabolic processes in response to changes light availability. Understanding how signals from the internal circadian clock and external light availability are integrated to control metabolic shifts will be important for engineering cyanobacteria for production in natural outdoor environments. This study has assessed how "knowing" the correct time of day, via the circadian clock, affects metabolic changes when a cyanobacterium goes through a dark-to-light transition. Our data show that the circadian clock plays an important role in inhibiting activation of the oxidative pentose phosphate pathway in the morning. Synechococcus elongatus PCC 7942 is a genetically tractable model cyanobacterium that has been engineered to produce industrially relevant biomolecules and is the best-studied model for a prokaryotic circadian clock. However, the organism is commonly grown in continuous light in the laboratory, and data on metabolic processes under diurnal conditions are lacking. Moreover, the influence of the circadian clock on diurnal metabolism has been investigated only briefly. Here, we demonstrate that the circadian oscillator influences rhythms of metabolism during diurnal growth, even though light-dark cycles can drive metabolic rhythms independently. Moreover, the phenotype associated with loss of the core oscillator protein, KaiC, is distinct from that caused by absence of the circadian output transcriptional regulator, RpaA (regulator of phycobilisome-associated A). Although RpaA activity is important for carbon degradation at night, KaiC is dispensable for those processes. Untargeted metabolomics analysis and glycogen kinetics suggest that functional KaiC is important for metabolite partitioning in the morning. Additionally, output from the oscillator functions to inhibit RpaA activity in the morning, and kaiC-null strains expressing a mutant KaiC phosphomimetic, KaiC-pST, in which the oscillator is locked in the most active output state, phenocopies a{Delta} rpaA strain. Inhibition of RpaA by the oscillator in the morning suppresses metabolic processes that normally are active at night, and kaiC-null strains show indications of oxidative pentose phosphate pathway activation as well as increased abundance of primary metabolites. Inhibitory clock output may serve to allow secondary metabolite biosynthesis in the morning, and some metabolites resulting from these processes may feed back to reinforce clock timing.
