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SYNOPSIS. Nine Crithidia spp., 2 Blastocrithidia spp., 3 Leptomonas spp. and 2 Trypanosoma spp. were tested for ability to synthesize methionine and lysine during growth. A prerequisite for methionine biosynthesis is an inordinately high level of folic acid (0.1 mg/100 ml) in the medium. Crithidia factor‐type unconjugated pteridines cannot spare this requirement. Since the methionine‐synthesis factor is still present after acid hydrolysis which destroys folic acid, the factor is either a breakdown product of folic acid or an impurity in the commercial product. All save C. fasciculata var. noelleri and C. from Syrphid could synthesize methionine from homocysteine thiolactone. None of the organisms synthesized lysine from α‐aminoadipic acid (AAA), thus ruling out the existence of the AAA pathway for lysine synthesis in the Trypanosomatidae. Nine of the organisms synthesized lysine from a mixture of LL‐ and meso‐α,e‐diaminopimelic acid. Since both LL‐ and meso‐DAP are intermediates in the biosynthesis of lysine by the DAP pathway (LL‐DAP→meso‐DAP→lysine) and since decarboxylation of either LL‐ or meso‐DAP could result in formation of lysine, pure meso‐DAP was tested and found active. Thus at least the terminal portion of the DAP pathway for lysine synthesis exists in these true animal cells. Statements about absence of ability to synthesize lysine in animal cells and consequent evolutionary interpretations will therefore require revision.

SYNOPSIS. Defined media for Leptomonas collosoma, L. mirabilis, and L. sp. from Dysdercus are described. In addition to factors required by Crithidia spp., including a source of Crithidia factor, all 3 organisms require glycine. Leptomonas collosoma and L. sp. from Dysdercus require choline. All trypanosomatids thus far studied require calcium pantothenate but L. collosoma has an especially large requirement. Leptomonas mirabilis but not L. sp. from Dysdercus can synthesize methionine from homocysteine thiolactone during growth. Homocysteine thiolactone is toxic to L. sp. from Dysdercus. Methionine plus vitamin B12 partly annul homocysteine thiolactone toxicity but only if glycine is also present.