Characterization of Streptomyces isolates causing colour changes of mural paintings in ancient Egyptian tombs.

Paintings in ancient Egyptian tombs often suffer colour changes due to microbial growth and colonization. Streptomyces strains were isolated from mural paintings of Tell Basta and Tanis tombs (East of Nile Delta, Egypt) and were identified using biochemical and molecular methods. The16S rDNA sequences data indicated that isolated strains were closely related to S. coelicolor, S. albidofuscus, S. ambofaciens, S. canarius, S. parvullus, S. corchorusii, S. albidofuscus and S. nigrifaciens. It could be shown that Streptomyces strains are involved on a large scale in the colour changes of paintings and stone support by producing a wide range of metabolites such as acids (oxalic, citric and sulphuric acids), biopigments of melanin, carotenoids, and hydrogen sulphide.

Cloning and characterization of the gene for phosphatidylcholine synthase.

Phosphatidylcholine (PC) is the major membrane-forming phospholipid in eukaryotes and can be synthesized by either of two pathways, the CDP-choline pathway or the methylation pathway. In prokaryotes only the methylation pathway was thought to occur. Recently, however, we could demonstrate (de Rudder, K. E. E., Sohlenkamp, C., and Geiger, O. (1999) J. Biol. Chem. 274, 20011-20016) that a second pathway for phosphatidylcholine biosynthesis exists in Sinorhizobium (Rhizobium) meliloti involving a novel enzymatic activity, phosphatidylcholine synthase, that condenses choline and CDP-diacylglyceride in one step to form PC and CMP. Using a colony autoradiography method we have isolated mutants of S. meliloti deficient in phosphatidylcholine synthase and which are no longer able to incorporate radiolabeled choline into PC. Complementation of such mutants with a sinorhizobial cosmid gene bank, subcloning of the complementing fragment, and sequencing of the subclone led to the identification of a gene coding for a presumptive CDP-alcohol phosphatidyltransferase. Amplification of this gene and its expression in Escherichia coli demonstrates that it codes for phosphatidylcholine synthase. Genomes of some pathogens (Pseudomonas aeruginosa and Borrelia burgdorferi) contain genes similar to the sinorhizobial gene (pcs) for phosphatidylcholine synthase. Although pcs-deficient S. meliloti knock-out mutants show wild type-like growth and lipid composition, they are unable to perform rapid PC biosynthesis that normally is achieved via the phosphatidylcholine synthase pathway in S. meliloti wild type.

Characterization of Arabidopsis AtAMT2, a novel ammonium transporter in plants.

We have cloned and characterized the first member of a novel family of ammonium transporters in plants: AtAMT2 from Arabidopsis thaliana. AtAMT2 is more closely related to bacterial ammonium transporters than to plant transporters of the AMT1 family. The protein was expressed and functionally characterized in yeast. AtAMT2 transported ammonium in an energy-dependent manner. In contrast to transporters of the AMT1 family, however, AtAMT2 did not transport the ammonium analogue, methylammonium. AtAMT2 was expressed more highly in shoots than roots and was subject to nitrogen regulation.

Plant-exuded choline is used for rhizobial membrane lipid biosynthesis by phosphatidylcholine synthase.

Phosphatidylcholine is a major lipid of eukaryotic membranes, but found in only few prokaryotes. Enzymatic methylation of phosphatidylethanolamine by phospholipid N-methyltransferase was thought to be the only biosynthetic pathway to yield phosphatidylcholine in bacteria. However, mutants of the microsymbiotic soil bacterium Sinorhizobium (Rhizobium) meliloti, defective in phospholipid N-methyltransferase, form phosphatidylcholine in wild type amounts when choline is provided in the growth medium. Here we describe a second bacterial pathway for phosphatidylcholine biosynthesis involving the novel enzymatic activity, phosphatidylcholine synthase, that forms phosphatidylcholine directly from choline and CDP-diacylglycerol in cell-free extracts of S. meliloti. We further demonstrate that roots of host plants of S. meliloti exude choline and that the amounts of exuded choline are sufficient to allow for maximal phosphatidylcholine biosynthesis in S. meliloti via the novel pathway.