Fig. 6.

Increase of intracellular free Ca²⁺ concentration as revealed by recombinant obelin. Anabaena 7120 carrying pAM505-ob (A–D, F) or without pAM505-ob (E) was subjected to nitrogen step-down for 24 h (A–E) or for 12 h (F). Except for D, coelenterazine was added to 1 ml of cultures of Anabaena 7120 at an optical density (750 nm) of 0.3 to a final concentration of 2 μM and incubated for 30 min before they were observed with the fluorescence microscope. (A) Fluorescence image with green-light excitation that excited phycobiliproteins (PBP). (B and C) Fluorescence images with near-UV light excitation with the red fluorescence from chlorophyll and PBP unblocked (B) and blocked (C). (D) Fluorescence image in the absence of coelenterazine when excited with near-UV light. (E) Fluorescence image of the wild-type Anabaena 7120 without obelin in the presence of coelenterazine with near-UV light excitation. (F) Fluorescence image of Anabaena 7120 carrying pAM505-ob 12 h after the removal of combined nitrogen. The filament was excited with near-UV light, and the image was obtained as in C. (Bar, 10 μm.)