No abstract

An L-amino acid oxidase with high specifity for basic L-amino acids was isolated from the cyanobacterium Synechococcus PCC 7942, and the enzyme was partially characterized. This enzyme was compared to the previously described L-amino acid oxidase from Synechococcus PCC6301 (G. Wälzlein, A. E. Gau, and E. K. Pistorius, Z. Naturforsch. 43c, 5 4 5-553, 1988). In addition, photosystem II complexes were isolated from Synechococcus PCC 7942, and it could be shown that a 36 kDa polypeptide which crossreacts with the antiserum raised against the L-amino acid oxidase (50 kD a) is present in isolated PS II complexes from Synechococcus PCC 7942 as already shown to be the case for Synechococcus PC C 6301 (A. E. Gau, G. W älzlein, S. Gärtner, M. Kuhlmann, S. Specht, and E. K. Pistorius, Z. Naturforsch. 44c, 9 7 1 -9 7 5 , 1989). These results clearly show that in isolated photosystem II complexes from Synechococcus PCC 6301 as well as PCC 7942 a fourth polypeptide (besides D 1, D 2 and the manganese stabilizing protein) is present in the 30 kDa region and support our hypothesis suggesting that the water oxidizing enzyme is a separate protein (distinct from D 1 and D 2)

The authors previously reported the isolation and partial characterization of a periplasmically located dihydrolipoamide dehydrogenase (LPD) from the cyanobacterium Synechocystis sp. strain PCC 6803. In the present work the gene (IpdA; database accession number 248564) encoding the apoprotein of this LPD in Synechocystis PCC 6803 has been identified, sequenced and analysed. The IpdA gene codes for a protein starting with methionine, which is post-translationally removed. The mature protein contains an N-terminal serine and consists of 473 amino acids with a deduced molecular mass of 51 421 Da (including one FAD). The LPD is an acidic protein with a calculated isoelectric point of 5-17. Comparison of the amino acid sequence of the Synechocystis LPD with protein sequences in the databases revealed that the enzyme shares identities of 31-35% with all 18 LPDs so far sequenced and published. As a first step in determining the role of this cyanobacterial LPD, attempts were made to generate an LPD-free Synechocystis mutant by insertionally inactivating the IpdA gene with a kanamycin-resistance cassette. However, the selected transformants appeared to be heteroallelic, containing both the intact IpdA gene and the IpdA gene inactivated by the drug-resistance cassette. The heteroallelic mutant studied, which had about 50% of the wildtype LPD activity, caused acidification of the growth medium. Growth over a prolonged time was only possible after an increased buffering of the medium. Since it is reported in the literature that inactivation of the pyruvate dehydrogenase complex (PDC) leads to acidosis, a function of the LPD in a cytoplasmic-membrane-associated PDC is conceivable.