EFFECT OF THE NITROGEN-SOURCE ON PHYCOBILIPROTEIN SYNTHESIS AND CELL RESERVES IN A CHROMATICALLY ADAPTING FILAMENTOUS CYANOBACTERIUM

Cyanobacteria can utilize nitrate or ammonium as a source of fixed nit rogen for cell growth. In the filamentous Calothrix sp. strain PCC 760 1, these two sources of nitrogen differently influenced the phycobilip rotein composition of the phycobilisomes, the major light-harvesting a ntennae. When compared to nitrate, growth in the presence of ammonium resulted in intracellular steady-state levels 35% lower for phycoeryth rin and 46% higher for phycocyanin. Besides these differences in cell pigmentation, a rapid but transient accumulation of cyanophycin granul e polypeptide occurred in ammonium-grown cells, while these macromolec ules were not detected in cells grown with nitrate. In contrast, glyco gen reserves displayed a dynamic pattern of accumulation and disappear ance during cell growth which varied only slightly with the nitrogen s ource. The observed changes in cell pigmentation are reminiscent of th e phenomenon of complementary chromatic adaptation, in which green and red wavelengths promote the syntheses of phycoerythrin and phycocyani n-2, respectively. As in complementary chromatic adaptation, the regul ation of synthesis of phycoerythrin and phycocyanin-2 by the nitrogen source occurred mainly at the mRNA level. Moreover, the transcriptiona l start sites for the expression of the cpeBA and the cpc2 operons, wh ich respectively encode the two subunits of phycoerythrin and phycocya nin-2 were the same in cells grown in nitrate or ammonium, and identic al to those in green- and red-light-grown cells. The results of this s tudy suggest that acclimation to the spectral light quality and to the nitrogen source share some common regulatory elements.