An action spectrum for nucleic acid formation in an achlorophyllous mutant of Chlorella pyrenoidosa

Blue light-dependent regulation of cytoplasmic ribosomal RNA synthesis in Chlorella

Effect of blue and red light on ribosomal RNA synthesis in autotrophic synchronous cultures of Chlorella pyrenoidosa (strain 211-8b) is studied by pulse labeling experiments with tritiated guanosine. Nucleic acids were separated by electrophoresis on polyacrylamide gels. Compared with darkness and red light (679 nm), blue light (457 nm) of equal quantum flux (0.5-5x10(-10) Einstein cm-2 s-1) stimulates incorporation into ribosomal RNA. This blue light effect is observed in the cytoplasmic ribosomal RNA after 5 min of illumination, whereas the stimulation of chloroplast ribosomal RNA synthesis by blue light appears later. Maturation of chloroplast ribosomal RNA is slower than that of cytoplasmic ribosomal RNA. The blue light effect on the cytoplasmic ribosomal RNA formation does not require chloroplast RNA or protein synthesis as shown by inhibitor studies with rifampicin or lincomycin. The blocking of cytoplasmic protein synthesis by cycloheximide inhibits the blue light effect on ribosomal RNA formation. It is concluded that the cytoplasmic ribosomal RNA transcription is controlled by a blue light sensitive system.

Photoregulation of transfer and 5S ribosomal RNA synthesis in Chlorella

The effect of blue and red light on the synthesis of transfer and 5S ribosomal RNA in autotrophic cultures of Chlorella pyrenoidosa was studied by pulse labeling experiments with tritiated guanosine. Compared with darkness or red light (679 nm), blue light (457 nm) of low intensities (quantum flux: 0.5-5×10(-10) mol photons cm(-2) s(-1)) stimulated incorporation of guanosine into transfer and 5S ribosomal RNA within the first 5 min of illumination. This blue light effect was abolished by cycloheximide, an inhibitor of protein synthesis on 80S ribosomes, but not by rifampicin, an inhibitor of chloroplastic transcription, nor by lincomycin, an inhibitor of chloroplastic translation. The rifampicin-insensitive synthesis of transfer and 5S ribosomal RNA was nuclear transcription as shown by RNA-DNA hybridization. The blue light effect on nuclear RNA synthesis was not inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethyl-urea, an inhibitor of photosynthesis electron transport. Further evidence for a photosynthesis-independent photocontrol of RNA synthesis was provided by experiments with the colorless mutant 125a of Chlorella vulgaris. Blue light stimulated incorporation of guanosine into cytoplasmic 25S and 18S ribosomal RNA as well as into transfer and 5S ribosomal RNA, whereas incorporation in red light was the same as that of the dark control.

[Effect of blue and red light on the synthesis of ribosomal RNA in Chlorella]

In autotrophic cultures of Chlorella pyrenoidosa (strain 211-8b) incorporation of tritiated guanosine and uridine into ribosomal RNA is stimulated by light. Blue light of wavelengths around 457 nm is considerably more effective than red light around 679 nm (5-10(-10) Einstein cm-2 sec-1 for both). This effect can be demonstrated for young daughter cells (at the end of the dark period) and for older cells (at the end of the light period). It is shown to depend on a regulation of rRNA-synthesis. The blue light dependent enhancement of incorporation is more pronounced in the cytoplasmic rRNA (25 and 18 s) than in the chloroplast rRNA (23 and 16 s). Blue light of low intensity (1-10(-10) Einstein cm-2 sec-1) has nearly the same effectivity as the fivefold intensity, whereas red light of equal quantum fluxes enhances incorporation only slightly compared with the dark control. The blue light dependent enhancement of rRNA-synthesis continues in the following darkness in contrary to that caused by red light. This enhancement is also found in DCMU-poisened cultures. In contrast to this, is red light in presence of DCMU, incorporation into rRNA is nearly the same as in dark. It is concluded that the regulation of rRNA-synthesis in red light is closely connected to complete photosynthesis, while in blue light an additional regulation takes place independent of photosynthesis.

[Promoting effect of blue light on the production of carotenoids in a yellow chlorella mutant]

In growing as well as in resting cells of a chlorophyll-free yellow mutant of Chlorella vulgaris (211-11h/20) synthesis of carotenoids is enhanced by blue light. Permanent irradiation is necessary to maintain the effect (Fig. 2). At wavelengths around 454 nm the additional carotenoid production is half-saturated at about 1000 erg cm(-2)s(-1) and saturated at about 4000 erg cm(-2)s(-1) (Fig. 3). An action spectrum exhibits highest efficiency of wavelengths around 465 and 370 nm, a minimum near 400 nm and inefficacy of yellow, red and far-red light (Fig. 4). This wavelength dependence resembles those of light enhanced O2-uptake, carbohydrate consumption and gain of organically bound nitrogen by the organism.