Photoassimilation of aromatic compounds by Rhodobacter sphaeroides OU5

Abstract

Purple bacteria metabolize aromatic compounds either by light dependent degradation of benzene ring, supporting growth by providing carbon source/electron donors or through transformations when they cannot cleave the benzene ring. Rhodobacter sphaeroides OU5, a purple non-sulfur bacterium lacks the capability to degrade aromatic compounds but has the ability to transform some of the aromatic compounds. The present thesis work extends an in-depth study on the metabolic potential of Rhodobacter sphaeroides OU5 in utilization of few aromatic compounds, though not as growth substrates. Utilization of aromatic compounds as sole source of carbon or as electron donors by Rba. sphaeroides OU5 could not be demonstrated. However, assimilation of trans-cinnamate, 4-hydroxycinnamate, 3, 4- dihydroxycinnamate and 4-hydroxybenzoate was observed in Rba. sphaeroides OU5. Inhibition in growth of Rba. sphaeroides OU5 and on the activity of 3-deoxy D-arabino-heptulosanate 7-phosphate (DAHP) synthase, (key enzyme in the shikimate pathway that leads to the biosynthesis of aromatic amino acids) in presence of aromatic compounds like transcinnamate and hydroxycinnamate was demonstrated. trans-Cinnamate assimilation was light dependent and was observed by both growing and resting cells of Rba. sphaeroides OU5. Studies with whole cells and cell free extracts of Rba. sphaeroides OU5 indicate the transformation of trans-cinnamate to L-phenylalanine and Ltryptophan with intermediate phenylpyruvate. Involvement of the enzyme phenylalanine ammonia lyase (PAL) in the conversion of transcinnamate to L-phenylalanine could not be demonstrated in Rba. sphaeroides OU5. A ~ 43kD protein catalyzing this irreversible reaction was purified to electrophoretic homogeneity and was characterized from trans-cinnamate induced culture of Rba. sphaeroides OU5. In addition to trans-cinnamate, Rba. sphaeroides OU5 assimilated hydroxycinnamates like 4-hydroxycinnamate, 3, 4-dihydroxycinnamate which are transformed to L-tyrosine and 3, 4-dihydroxyphenylalanine (DOPA), respectively. Among benzoates tested, 4-hydroxybenzoate was assimilated by Rba. sphaeroides OU5. In conclusion, a novel metabolism of aromatic compound assimilation, with few biochemical modifications to an aromatic amino acid was suggested in Rba. sphaeroides OU5.