ELECTRON-TRANSPORT REGULATES CELLULAR-DIFFERENTIATION IN THE FILAMENTOUS CYANOBACTERIUM CALOTHRIX

Differentiation of the filamentous cyanobacteria Calothrix sp strains PCC 7601 and PCC 7504 is regulated by light spectral quality. Vegetati ve filaments differentiate motile, gas-vacuolated hormogonia after tra nsfer to fresh medium and incubation under red light. Hormogonia are t ransient and give rise to vegetative filaments, or to heterocystous fi laments if fixed nitrogen is lacking. If incubated under green light a fter transfer to fresh medium, vegetative filaments do not differentia te hormogonia but may produce heterocysts directly, even in the presen ce of combined nitrogen. We used inhibitors of thylakoid electron tran sport (3-[3,4-dichlorophenyl]-1,1-dimethylurea and 2,5-dibromo-3-methy l-6-isopropyl-p-benzoquinone) to show that the opposing effects of red and green light on cell differentiation arise through differential ex citations of photosystems I and II. Red light excitation of photosyste m I oxidizes the plastoquinone pool, stimulating differentiation of ho rmogonia and inhibiting heterocyst differentiation. Conversely, net re duction of plastoquinone by green light excitation of photosystem II i nhibits differentiation of hormogonia and stimulates heterocyst differ entiation. This photoperception mechanism is distinct from the light r egulation of complementary chromatic adaptation of phycobilisome const ituents. Although complementary chromatic adaptation operates independ ently of the photocontrol of cellular differentiation, these two regul atory processes are linked, because the general expression of phycobil iprotein genes is transiently repressed during hormogonium differentia tion. In addition, absorbance by phycobillsomes largely determines the light wavelengths that excite photosystem II, and thus the wavelength s that can imbalance electron transport.