A GLOBAL 2-COMPONENT SIGNAL-TRANSDUCTION SYSTEM THAT INTEGRATES THE CONTROL OF PHOTOSYNTHESIS, CARBON-DIOXIDE ASSIMILATION, AND NITROGEN-FIXATION

Photosynthesis, biological nitrogen fixation, and carbon dioxide assim ilation are three fundamental biological processes catalyzed by photos ynthetic bacteria. In the present study, it is shown that mutant strai ns of the nonsulfur purple photosynthetic bacteria Rhodospirillum rubr um and Rhodobacter sphaeroides, containing a blockage in the primary C O2 assimilatory pathway, derepress the synthesis of components of the nitrogen fixation enzyme complex and abrogate normal control mechanism s. The absence of the Calvin-Benson-Bassham (CBB) reductive pentose ph osphate CO2 fixation pathway removes an important route for the dissip ation of excess reducing power, Thus, the mutant strains develop alter native means to remove these reducing equivalents, resulting in the sy nthesis of large amounts of nitrogenase even in the presence of ammoni a. This response is under the control of a global two-component signal transduction system previously found to regulate photosystem biosynth esis and the transcription of genes required for CO2 fixation through the CBB pathway and alternative routes. In addition, this two-componen t system directly controls the ability of these bacteria to grow under nitrogen-fixing conditions. These results indicate that there is a mo lecular link between the CBB and nitrogen fixation process, allowing t he cell to overcome powerful control mechanisms to remove excess reduc ing power generated by photosynthesis and carbon metabolism. Furthermo re, these results suggest that the two-component system integrates the expression of genes required for the three processes of photosynthesi s, nitrogen fixation, and carbon dioxide fixation.