The reduction of nitrous oxide to dinitrogen by Escherichia coli.

Escherichia coli K12 reduces nitrous oxide stoichiometrically to molecular nitrogen with rates of 1.9 mumol/h x mg protein. The activity is induced by anaerobiosis and nitrate. N2-formation from N2O is inhibited by C2H2 (Ki approximately 0.03 mM in the medium) and nitrite (Ki = 0.3 mM) but not by azide. A mutant defective in FNR synthesis is unable to reduce N2O to N2. The reaction in the wild type could routinely be followed by gas chromatography and alternatively by mass spectrometry measuring the formation of 15N2 from 15N2O. The enzyme catalyzing N2O-reduction in E. coli could not be identified; it is probably neither nitrate reductase nor nitrogenase. E. coli does not grow with N2O as sole respiratory electron acceptor. N2O-reduction might not have a physiological role in E. coli, and the enzyme involved might catalyze something else in nature, as it has a low affinity for the substrate N2O (apparent Km approximately 3.0 mM). The capability for N2O-reduction to N2 is not restricted to E. coli but is also demonstrable in Yersinia kristensenii and Buttiauxella agrestis of the Enterobacteriaceae. E. coli is able to produce NO and N2O from nitrite by nitrate reductase, depending on the assay conditions. In such experiments NO2- is not reduced to N2 because of the high demand for N2O of N2O-reduction and the inhibitory effect of NO2- on this reaction.

Structural organization of the chloroplast genome of the chromophytic alga Vaucheria bursata.

The chloroplast genome of the chromophytic alga Vaucheria bursata has been characterized by restriction site and gene mapping analysis. It is represented by a circular molecule 124.6 kb in size. An inverted sequence duplication (IR) not larger than 5.85 kb carries the rRNA genes and separates two single-copy regions of 64.6 kb and 48.3 kb from one another. The Vaucheria plastid genome exists in two equimolar isomers which is due to intramolecular flip-flop recombination within the IR sequences. The coding sites for 21 structural and soluble proteins have been mapped on both single-copy regions using heterologous gene sequences as probes. Although the overall gene order is found to be rearranged when compared with other chromophytic algal and land plant chloroplast genomes, most of the transcriptional units of cyanobacteria and land plant chloroplast genomes appear to be conserved. The phylogenetic implications of these findings are further discussed.

Structural organization and evolution of the plastid genome of Vaucheria sessilis (Xanthophyceae).

The plastid DNAs of 18 Vaucheria sessilis strains from various habitats in western Europe were digested with the restriction endonucleases Eco RI, Sal I, Bam HI and Pvu II. Their restriction patterns showed variable fragment divergencies. Two main groups of plastid genomes were recognized, which were substantiated by morphological features. The differences among the restriction patterns could be attributed to the loss or appearance of restriction sites and to minor size variations caused by deletions/insertions. The Sal I and Bam HI restriction sites which together discriminate six different plastid genomes were mapped on the circular molecule of 124 kilobase paris (kbp). The plastid genomes of several Vaucheria sessilis strains were shown to exist in two inversion isomers caused by intramolecular recombination within the inverted repeat segments.