The participation of the Shemin and C5 pathways in 5-aminolaevulinate and chlorophyll formation in higher plants and facultative photosynthetic bacteria.
Chlorophyll and bacteriochlorophyll formation from 14C-labelled precursors was studied during the illumination of etiolated maize leaves excised from dark-grown seedlings and in cell suspensions of respiring, dark-, aerobically-grown Rhodopseudomonas spheroides adapting to the photosynthetic state in the light under anaerobic conditions. It was found that 1-14C-labelled glutamate and 2-oxoglutarate were incorporated into the tetrapyrrole moieties of chlorophyll and bacteriochlorophyll. This suggests that the C5 pathway of tetrapyrrole biosynthesis operates in both Zea mays and R. spheroides since in the alternative Shemin pathway the label would have been lost as 14CO2 during the formation of succinyl-CoA prior to the condensation with glycine to form 5-aminolaevulinate. It was also found that 5(-14)C-labelled glutamate and 2-oxoglutarate were incorporated into these chlorophylls which is consistent with the operation of both the C5 and Shemin pathways. That the Shemin pathway is also involved was confirmed by the incorporation of [2(-14)C]glycine into both chlorophylls. None of these substrates were incorporated into the phytol moieties of either plant or bacterial chlorophyll or into the carotenoids. However, when [1(-14)C]acetate was added to greening maize leaves not only the tetrapyrrole and phytol moieties were labelled but also the carotenoids: the labelling of these lipids is consistent with their formation from acetate via the isopentenyl pyrophosphate pathway. By comparing the incorporation of [1(-14)C]2-oxoglutarate with that of [5(-14)C]2-oxoglutarate the approximate relative contribution of each pathway to chlorophyll biosynthesis was determined. In maize leaves both pathways contributed almost equally but in R. spheroides the contribution by the Shemin and C5 pathways was 90 and 10%, respectively.