The genetics of heterocyst metabolism in the cyanobacterium, Anabaena sp. strain PCC 7120.

摘要

Anabaena sp. strain PCC 7120 is a filamentous cyanobacterium that is capable both of fixing nitrogen into organic compounds and of oxygenic photosynthesis. When deprived of combined nitrogen, approximately every tenth cell along the filament differentiates into a nitrogen-fixing cell called a heterocyst. The heterocysts supply the filament with combined nitrogen while the vegetative cells perform photosynthesis and supply the heterocysts with fixed carbon. The morphological and biochemical changes that take place during heterocyst differentiation provide the nitrogen-fixing enzyme nitrogenase with an anaerobic environment and the large supply of reductant and ATP that it requires.; Cytochrome c oxidase is the major terminal oxidase in cyanobacterial respiration. In this study, two cytochrome c oxidase operons (ctaCDE and coxBAC) were identified in Anabaena sp. strain PCC 7120. The expression of ctaCDE is heterocyst-specific and increases 20–25-fold during heterocyst differentiation, while the expression of coxBAC occurs in both cell types. Insertional mutagenesis of the ctaD gene results in three strains that are genetically identical at the ctaD insertion site, but that have different phenotypes. Two of these strains are viable in the absence of combined nitrogen and have some nitrogenase activity. One strain has no nitrogenase activity and dies rapidly upon nitrogen step-down. It is proposed that the two strains with greater viability have acquired suppressor mutations.; The patB gene in Anabaena sp. strain PCC 7120 encodes a protein with a bacterial type ferredoxin domain at the amino-terminus and a helix-turn-helix domain at the carboxy-terminus. Expression of patB is induced during nitrogen starvation, specifically in heterocysts. A frameshift mutation in the carboxy-terminal domain of the protein results in a growth defect, formation of multiple contiguous heterocysts, and reduced nitrogenase activity. However, a deletion mutant accumulates fragmented filaments, has almost no detectable nitrogenase activity, and dies much more rapidly than the frameshift mutant, indicating that the ferredoxin domain plays an important role in PatB function. Ectopic expression of patB in all cells of the filament results in altered cell morphology. Possible roles for PatB in the function of nitrogenase or in the regulation of nitrogen assimilation are discussed.