Metabolism of l-phenylalanine and l-tyrosine by the phototrophic bacteriumRhodobacter capsulatus

The phototrophic bacterium Rhodobacter capsulatus utilizes the aromatic ami no acids L-phenylalanine and L-tyrosine as nitrogen source. L-Phenylalanine is hydroxylated to L-tyrosine, which is further converted into p-hydroxyph enyl pyruvate (pHPP) by a transamination reaction. The bacterium is unable to grow at the expense of these amino acids as the sole carbon source, alth ough it is able to degrade them to homogentisate, probably by unspecific hy droxylation reactions. Metabolization of L-phenylalanine or L-tyrosine as n itrogen source requires phototrophic growth conditions and does not produce free ammonium inside the cells. A low aminotransferase activity with 2-oxo glutarate and L-tyrosine as substrates can be detected in crude extracts of R. capsulatus. Uptake of both amino acids by R. capsulatus was completely inhibited by ammonium addition, which also prevents aminotransferase induct ion.