Anoxygenic phototrophs, like Rhodopseudomonas palustris, can convert light energy 15 and electrons from organic waste into H 2 gas, a potential biofuel. During phototrophic 16 growth on organic compounds the CO 2-fixing Calvin cycle competes against H 2 17 production for electrons. Here we address why genetically disrupting the CO 2-fixing 18 enzyme ribulose 1,5-bisphosphate carboxylase (Rubisco), increases the H 2 yield but not 19 the specific H 2 production rate. We hypothesized that remaining upstream 20 phosphoribulokinase (PRK) activity negatively impacts growth and thereby the specific 21 H 2 production rate, likely due to the accumulation of ribulose-1,5-bisphosphate, the 22 substrate for Rubisco. In agreement with our hypothesis, deletion of PRK resulted in 23 proportional increases to both the H 2 yield and the specific production rate. Thus, even 24 though Rubsico is traditionally a more common target to eliminate Calvin cycle activity 25 we propose PRK as a favorable alternative to avoid undesirable pleiotropic effects.