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

Abstract

Cultures of R. spheroides grown with suboptimum amounts of biotin grow and form porphyrins and bacterlochlorophyll on subsequent illumination in a medium containing a-oxoglutarate and glycine with the addition of biotin. Growth and formation of both pigments is prevented by p-fluorophenylalanine, 8-azaguanine and 5-bromouracil. Growth and formation of bacteriochlorophyll are also inhibited by both D- and L-ethionine. The production of porphyrin is stimulated even when biotin is not added to the illumination medium. Ethlonine has the same effect on organisms grown in a complete medium. The amount of porphyrin formed in the presence of ethlonine is at least twice as great as the sum of the porphyrin and bacteriochlorophyll formed in its absence. Of a large number of compounds tested only DL-threonlne resembles ethlonine in stimulating porphyrin production and decreasing the synthesis of bacterlochlorophyll. The effects of both ethionlne and threonine are reversed by addition of methionine or homocystelne thiolactone. The effect of threonine is also reversed by the addition of homoserine. These results suggested that methionine might be specifically concerned in the biosynthesis of bacteriochlorophyll and particularly of the methyl ester group. Organisms were illuminated in the presence of [Me-14C] methionine and bacteriochlorophyll was isolated and degraded to yield methanol, phytol and bacteriochlorin. The bacteriochlorophyll possessed high radioactivity which was almost exclusively in the methyl ester group. In 1 experiment total carotenoids were isolated and found to have a molar radioactivity of the same order of magnitude as the bacterlochlorophyll. The significance of these results is discussed. It is concluded that ethlonine inhibits the synthesis of bacteriochlorophyll by interfering with the formation of the methyl ester group, and that threonine, by virtue of the fact that it shares a common blosynthetic pathway with methionine, inhibits the synthesis of the latter compound.