Studies on the biosynthesis of porphyrin and bacteriochlorophyll by Rhodopseudomonas spheroides. 2. The effects of ethionine and threonine

Gibson, Neuberger & Tait (1962) mentioned briefly that, when ethionine was added to suspen- sions of Rhodop8eudomona8 8pheroides illuminated in 'mixture I' of Lascelles (1956), large amounts of porphyrin accumulated in the medium, although growth and bacteriochlorophyll synthesis were in- hibited.This was rather surprising since other inhibitors of growth prevented the formation of porphyrin and bacteriochlorophyll.In the present paper the effect of ethionine is described in more detail.Of a large number of other compounds only threonine was found to have a similar effect.In the presence of ethionine or threonine the synthesis of bacteriochlorophyll can be restored and porphyrin excretion reduced by methionine or homocysteine.These results suggested that ethionine exerts its effect by interfering with the synthesis or utiliza- tion of methionine.This was confirmed when it was found that the methyl ester group of bacterio- chlorophyll is formed from the methyl group of methionine.Some of these results have been briefly presented earlier (Neuberger, 1961; Gibson, Matthew, Neuberger & Tait, 1961).