An ATP-sensitive phosphoketolase regulates carbon fixation in cyanobacteria

Regulation of CO 2 fixation in cyanobacteria is important both for the organism and global carbon balance. Here we show that phosphoketolase in Synechococcus elongatus PCC7942 ( Se XPK) possesses a distinct ATP-sensing mechanism, where a drop in ATP level allows Se XPK to divert precursors of the RuBisCO substrate away from the Calvin–Benson–Bassham cycle. Deleting the Se XPK gene increased CO 2 fixation particularly during light–dark transitions. In high-density cultures, the Δxpk strain showed a 60% increase in carbon fixation and unexpectedly resulted in sucrose secretion without any pathway engineering. Using cryo-EM analysis, we discovered that these functions were enabled by a unique allosteric regulatory site involving two subunits jointly binding two ATP, which constantly suppresses the activity of Se XPK until the ATP level drops. This magnesium-independent ATP allosteric site is present in many species across all three domains of life, where it may also play important regulatory functions. Marine cyanobacteria contribute to global carbon balance by fixing CO 2 and the shift between CO 2 fixation and ATP production requires fine-tuning its metabolic fluxes to light–dark cycles. These cycles can be very short in marine environments due to sea currents and fast adaptation is key to avoid futile cycles. In this study, Lu et al. provide a mechanistic insight into how this process is tightly regulated.