An ATP-sensitive phosphoketolase regulates carbon fixation in cyanobacteria

Abstract 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.