Identifying Regulatory Changes to Facilitate Nitrogen Fixation in the Nondiazotroph <i>Synechocystis</i> sp. PCC 6803

The incorporation of biological nitrogen fixation into a nondiazotrophic photosynthetic organism provides a promising solution to the increasing fixed nitrogen demand, but is accompanied by a number of challenges for accommodating two incompatible processes within the same organism. Here we present regulatory influence networks for two cyanobacteria, <i>Synechocystis</i> PCC 6803 and <i>Cyanothece</i> ATCC 51142, and evaluate them to co-opt native transcription factors that may be used to control the <i>nif</i> gene cluster once it is transferred to <i>Synechocystis</i>. These networks were further examined to identify candidate transcription factors for other metabolic processes necessary for temporal separation of photosynthesis and nitrogen fixation, glycogen catabolism and cyanophycin synthesis. Two transcription factors native to <i>Synechocystis</i>, LexA and Rcp1, were identified as promising candidates for the control of the <i>nif</i> gene cluster and other pertinent metabolic processes, respectively. Lessons learned in the incorporation of nitrogen fixation into a nondiazotrophic prokaryote may be leveraged to further progress the incorporation of nitrogen fixation in plants.