UC San Diego

Cyanobacteria are the architects of photosynthesis, and have evolved under the constant evolutionary pressure of predictable light and dark cycles. Consequently, they are the only bacterial lineage known to have a fully functional circadian clock. Due to their ubiquitous presence over the earth, and their growing importance in human health and bioindustry, a complete understanding of how their core metabolism interacts with natural diel cycles and circadian rhythms is a critical knowledge area that can be broadly applied to the understanding of cyanobacteria in both natural and artificial environments. Unfortunately, few studies have addressed the metabolic and physiological influence of the cyanobacterial circadian clock under a natural diel cycle. Here we use the bacterium Synechococcus elongatus PCC 7942 to address critical knowledge gaps regarding how cyanobacteria integrate signals from light and circadian rhythms to orchestrate core metabolic processes. S. elongatus is an ideal model for this study as it has emerged as a proven metabolic engineering platform, is currently the most well studied model of the cyanobacterial circadian clock, and is ubiquitous in fresh water environments. Using a combinatorial approach employing untargeted metabolomics, we have directly demonstrated that the S. elongatus circadian clock is an important control mechanism for central carbon metabolism during light-dark transitions. Specifically, we show that the core oscillator exerts a repressive effect on the downstream circadian response regulator RpaA. In the morning, the repression of RpaA by the clock is important for suppressing the oxidative pentose phosphate pathway (OPPP), which normally performs carbon compound catabolism at night, such that carbon from photosynthesis can flow to secondary biosynthetic pathways. In turn, this shows that OPPP activation is highly dependent on RpaA activity. Also, the activation of the OPPP is critical for survival at night, and we have shown that without RpaA mediated activation cell death ensues due to the inability to generate cellular reducing power. Characterization of the RpaA diurnal lethality phenotype has not only demonstrated the importance of diurnal reductant balance in S. elongatus, but also provides a new understanding as to why the circadian clock confers a fitness advantage to cyanobacteria under diurnal growth conditions.