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SUMMARY Chlorophyll formation in non-growing etiolated cells of Euglena gracilisvar. bacillaris is inhibited by certain antimetabolite analogues of purines, pyrimidines and amino acids. The inhibitory effects of bromo-and nitrouracils were annulled by uracil and thymine; those of fluoro-and thio-uracils were not. Ethionine inhibition was completely annulled by methionine. All the base analogues were more inhibitory for the dark growth of the alga than in the light and an adaptation to these antimetabolites was apparent during growth. The growth inhibitory effects of some of these base analogues was annulled by several Krebs's cycle intermediates; the latter in themselves stimulated growth of photosynthesizing euglenas although they were not utilized in the dark with any degree of efficiency.Long-term exposure of non-proliferating euglenas, green or etiolated, to 5-fluoro-uracil had a profound effect on the chloroplast integrity of the organisms. Such euglenas on further subculture in an adequate growth medium were permanently bleached. The effect of 5-bromo-uracil, though similar, was less marked.

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SUMMARYThe formation of chlorophyll in non-proliferating etiolated cells of Euglena gracilis var. bacillaris is inhibited by some antimetabolite analogues of vitamins. The cell size and cell mass of light-grown euglenas is considerably increased in vitamin B 12 deficiency. The inhibitory effect of 2,6-diaminopurine on chlorophyll formation in non-proliferating euglenas was not annulled by vitamin B 12 ; 6-mercaptopurine, sulphanilamide and benzimidazole were without effect. Isoniazide inhibition is not reversed by niacin; surprisingly, the vitamin itself is markedly inhibitory. The inhibitory effect of niacin, however, is prevented by pyridine-3-sulphonate. Niacin and its analogues were more inhibitory to growth in the dark than in the light. The inhibition of growth by niacin in the light is annulled appreciably by either glucose or pyruvate, or an overwhelming concentration of vitamin B 12. Aminopterin, desoxypyridoxine and 2-chloro-p-aminobenzoic acid do not have any effect on chlorophyll synthesis in non-proliferating euglenas ; the last mentioned inhibits the growth of the alga more in the dark than in the light. Thiamine deficiency inhibits growth ; such sub-optimally grown euglenas also synthesize less chlorophyll per cell on subsequent illumination under non-proliferation conditions.The negative growth response of light-grown Euglena gracilis var. bacillaris to niacin suggests a microbiological method of estimating this vitamin in biological materials and pharmaceutical preparations up to concentrations of 70 ,ug./ml. in the growth medium. The 50 % inhibition level of niacin in the light is 46 pg./ml.

SYNOPSIS. 8‐Hydroxyquinoline (8‐HQ) and cysteine markedly inhibit the synthesis of chlorophylls in non‐proliferating etiolated Euglena gracilis var. bacillaris on illumination. This is thought to be due, at least to some extent, to the binding of Mg2+ ions in the non‐proliferation medium, but largely as a consequence of intracellular chelating action. The effect of 8‐HQ could be reversed by the presence of metal ions higher in the Mellor‐Malley series than Mg2+, e.g., Zn2+ and Co2+. Sodium diethyldithiocarbamate (Na‐DDG) and ethylenediaminetetraacetic acid (EDTA) have no effect on chlorophyll synthesis in non‐proliferation conditions. Growth in the light is more affected than growth in the dark by all these chelating agents, including Na‐DDG and EDTA. This is due to the differential requirement of Mg2+ under these 2 growth conditions.