INVOLVEMENT OF 2 DIFFERENT PHOTORECEPTORS IN THE LIGHT REGULATION OF GLUTAMINE-SYNTHETASE ACTIVITY IN A CHLOROPHYLL-FREE CHLORELLA MUTANT

Glutamine synthetase (EC 6.3.1.2) activity of a non-photosynthetic mut ant of Chlorella kessleri is markedly enhanced under blue and slightly increased under red light. In both cases, the effect is largest after 6 h of irradiation. In blue light, saturation is reached at about 10 mu E m(-2) s(-1); in red light, it is not even indicated at 62 mu E m( -2) s(-1). Semilogarithmic plots of both intensity dependencies reveal different slopes, indicating envolvement of two separate photorecepto rs. This feature is supported by different effects of pulse irradiatio n: The response to 15 min of red light irradiation (lambda(max) 650 nm ) increases in subsequent darkness. It reaches the same value as in co ntinuous light after 6 h. The response to 15 min of blue light irradia tion (lambda(max) 441 nm) increases also in subsequent darkness. Howev er, after 6 h it reaches only 30% of the value obtained by continuous irradiation. It is concluded that, glutamine synthetase of Chlorella i s controlled by two different photoreceptors both independent of photo synthesis. There is evidence of two forms of glutamine synthetase, the intracellular distribution and specific light regulations of these ar e discussed.