The Sakaguchi Reaction Product Quenches Phycobilisome Fluorescence, Allowing Determination of the Arginine Concentration in Cells of Anabaena Strain PCC 7120

Abstract

The filamentous cyanobacterium Anabaena fixes nitrogen in specialized cells called heterocysts. The immediate product of fixation, ammonia, is known to be assimilated by addition to glutamate to make glutamine. How fixed nitrogen is transported along the filament to the 10 to 20 vegetative cells that separate heterocysts is unknown. N-fixing heterocysts accumulate an insoluble polymer containing aspartate and arginine at the cell poles. Lockau's group has proposed that the polymer is degraded at the poles to provide a mobile carrier, arginine, to the vegetative cells (R. Richter, M. Hejazi, R. Kraft, K. Ziegler, and W. Lockau, Eur. J. Biochem. 263:163-169, 1999). We wished to use the Sakaguchi reaction for arginine to determine the relative cellular concentration of arginine along the filament. At present, the methods for measuring absorption of the Sakaguchi reaction product at 520 nm are insufficiently sensitive for that purpose. However, that product quenches the fluorescence of phycobiliproteins, which we have adapted to a determination of arginine. Our results are consistent with the proposal that arginine is a principal nitrogen carrier from heterocysts to vegetative cells in Anabaena.

Inactivation of agmatinase expressed in vegetative cells alters arginine catabolism and prevents diazotrophic growth in the heterocyst‐forming cyanobacterium Anabaena

“…). This dipeptide appears to be an important vehicle for the transfer of nitrogen from heterocysts to vegetative cells, consistent with the possible existence of a gradient of arginine or an arginine‐containing compound in the diazotrophic filament (Ke and Haselkorn ). These results imply an active catabolism of arginine (and aspartate) in the vegetative cells of the diazotrophic filament.…”

“…As described in the Introduction, we have recently found that in the diazotrophic filament of Anabaena, the second step of cyanophycin degradation, hydrolysis of b-aspartyl-arginine, takes place mainly in vegetative cells (Burnat et al 2014). This dipeptide appears to be an important vehicle for the transfer of nitrogen from heterocysts to vegetative cells, consistent with the possible existence of a gradient of arginine or an arginine-containing compound in the diazotrophic filament (Ke and Haselkorn 2013). These results imply an active catabolism of arginine (and aspartate) in the vegetative cells of the diazotrophic filament.…”

Compartmentalized cyanophycin metabolism in the diazotrophic filaments of a heterocyst-forming cyanobacterium

“…Although the exact equation of intercellular nutrient exchange in the diazotrophic Anabaena filament is unknown, movement of sucrose, glutamate, and alanine from vegetative cells to heterocysts and of glutamine and β-aspartyl-arginine from heterocysts to vegetative cells could result in a net transfer of reduced carbon to heterocysts and of fixed nitrogen to vegetative cells. A possible gradient of arginine or of an argininecontaining compound in the diazotrophic filament of Anabaena has been noted (19).…”

“…Cyanophycin synthesis after nitrogen fixation has been suggested to serve an important function by removing from solution the products of nitrogen fixation, which could have a negative feedback effect on nitrogenase (18)(19)(20). Interestingly, this strategy appears to be generally used in nitrogen-fixing cyanobacteria independently of whether they are unicellular (36) or filamentous (37).…”

“…Although production of cyanophycin is not required for diazotrophic growth (16,17), its conspicuous presence in the heterocysts suggests a possible role of this cell inclusion in diazotrophy (18,19), for instance as a nitrogen buffer to balance nitrogen accumulation and transfer (20). Cyanophycin is synthesized by cyanophycin synthetase that adds both aspartate to the aspartate backbone of the polymer and arginine to the β-carboxyl group of aspartate residues in the backbone (21,22).…”