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STERLING EMERSON 



form at least as reduced as thiosulfate. At 35° it has no other requirement, 

 but at 25° it needs reduced sulfur, generally supplied as the amino acid 

 cysteine, and also tyrosine. When methionine is supplied as the source of sul- 

 fur at 25°, growth is strongly inhibited by choline. Under these conditions, 

 choline does not inhibit at 35°, but there is an unexpected stimulation in 

 growth by ^-aminobenzoic acid at that temperature. 



Mutant strains have been reported on two occasions which require either 

 choline or />-aminobenzoic acid — choline may be the source of the methyl 



to 

 ce 



HOURS 



Fig. 12.7 — Growth curves of suppressor mutant strain S-2 on minimal medium, on threo- 

 nine (5 mg/100 ml), on methionine, and on purines (5 mg/100 ml each adenine sulfate and 



guanine hydrochloride) at 35°. 



group of methionine. Strehler (1950) has reported a strain which requires 

 either methionine or /?-aminobenzoic acid. There is also a suggestion that 

 />-aminobenzoic acid may be involved in the metabolism of lysine. In Neuro- 

 spora this is suggested only because the double mutants between the sul- 

 fonamide-requiring strain and two different mutants which are unable to 

 synthesize lysine do not grow on any combination of growth factors we have 

 tried. In bacteria a strain has been found which requires either lysine or 

 /»-aminobenzoic acid as a growth factor (Koft el al., 1950), strengthening the 

 supposition of a similar interrelationship in Neurospora. 



These observations are referred to at this time because they indicate that 

 there are a large number of metabolic reactions that are in one way or an- 

 other related to the availability of />-aminobenzoic acid. These reactions 

 must themselves be interrelated in the sense that an upset in one of them 



