Structure and expression of the Chlorobium vibrioforme hemB gene and characterization of its encoded enzyme, porphobilinogen synthase

Influence of light intensity on chloroplast development and pigment accumulation in the wild-type and etiolated mutant plants of Anthurium andraeanum 'Sonate'

Seedlings of wild-type and etiolate mutant plants of Anthurium andraeanum cultivar 'Sonate' were treated for 15 d with different light intensities (20, 100, and 400 µmol·m^-2·s^-1) to analyze leaf plastid development and pigment content. Significant changes appeared in treated seedlings, including in leaf color, plastid ultrastructure, chloroplast development gene AaGLK expression, chlorophyll and anthocyanin contents, and protoplast shape. Wild-type and etiolated plants exhibited different plastid structures under the same light condition. The results suggest that light intensity is a crucial environmental factor influencing plastid development and leaf color formation in the A. andraeanum cultivar 'Sonate'.

Occurrence of two 5-aminolevulinate biosynthetic pathways in Streptomyces nodosus subsp. asukaensis is linked with the production of asukamycin

We report the results of cloning genes for two key biosynthetic enzymes of different 5-aminolevulinic acid (ALA) biosynthetic routes from Streptomyces. The genes encode the glutamyl-tRNAGlu reductase (GluTR) of the C5 pathway and the ALA synthase (ALAS) of the Shemin pathway. While Streptomyces coelicolor A3(2) synthesizes ALA via the C5 route, both pathways are operational in Streptomyces nodosus subsp. asukaensis, a producer of asukamycin. In this strain, the C5 route produces ALA for tetrapyrrole biosynthesis; the ALA formed by the Shemin pathway serves as a precursor of the 2-amino-3-hydroxycyclopent-2-enone moiety (C5N unit), an antibiotic component. The growth of S. nodosus and S. coelicolor strains deficient in the GluTR genes (gtr) is strictly dependent on ALA or heme supplementation, whereas the defect in the ALAS-encoding gene (hemA-asuA) abolishes the asukamycin production in S. nodosus. The recombinant hemA-asuA gene was expressed in Escherichia coli and in Streptomyces, and the encoded enzyme activity was demonstrated both in vivo and in vitro. The hemA-asuA gene is situated within a putative cluster of asukamycin biosynthetic genes. This is the first report about the cloning of genes for two different ALA biosynthetic routes from a single bacterium.

The X-ray structure of the plant like 5-aminolaevulinic acid dehydratase from Chlorobium vibrioforme complexed with the inhibitor laevulinic acid at 2.6 A resolution

5-Aminolaevulinic acid dehydratase (ALAD), an early enzyme of the tetrapyrrole biosynthesis pathway, catalyses the dimerisation of 5-aminolaevulinic acid to form the pyrrole, porphobilinogen. ALAD from Chlorobium vibrioforme is shown to form a homo-octameric structure with 422 symmetry in which each subunit adopts a TIM-barrel fold with a 30 residue N-terminal arm extension. Pairs of monomers associate with their arms wrapped around each other. Four of these dimers interact principally via their arm regions to form octamers in which each active site is located on the surface. The active site contains two invariant lysine residues (200 and 253), one of which (Lys253) forms a Schiff base link with the bound substrate analogue, laevulinic acid. The carboxyl group of the laevulinic acid forms hydrogen bonds with the side-chains of Ser279 and Tyr318. The structure was examined to determine the location of the putative active-site magnesium ion, however, no evidence for the metal ion was found in the electron density map. This is in agreement with previous kinetic studies that have shown that magnesium stimulates but is not required for activity. A different site close to the active site flap, in which a putative magnesium ion is coordinated by a glutamate carboxyl and five solvent molecules may account for the stimulatory properties of magnesium ions on the enzyme.

An unusual phylogenetic variation in the metal ion binding sites of porphobilinogen synthase

Porphobilinogen synthase (PBGS), which catalyzes the first common step in tetrapyrrole biosynthesis, contains a unique phylogenetic variation in the use of metal ions. Using sequence, structure, and enzymological information, this work codifies the phylogenetic segregation of metal utilization in PBGS from archaea, bacteria, and eucarya. All PBGS contain an active site metal binding sequence, determined herein to be either DXCXCX(Y/F)X(3)G(H/Q)CG or DXALDX(Y/F)X(3)G(H/Q)DG. The former dictates a requirement for zinc. Most PBGS that do not require zinc require magnesium and/or potassium instead. Most PBGS are also found to contain the binding determinants for an allosteric magnesium that resides outside the active site. The phylogenetic distribution of PBGS metal ion utilization suggests that the primordial PBGS required zinc and supports a hypothesis that the loss of the zinc site was concurrent with the advent of oxygenic photosynthesis.

Biochemistry, regulation and genomics of haem biosynthesis in prokaryotes

Haems are involved in many cellular processes in prokaryotes and eukaryotes. The biosynthetic pathway leading to haem formation is, with few exceptions, well-conserved, and is controlled in accordance with cellular function. Here, we review the biosynthesis of haem and its regulation in prokaryotes. In addition, we focus on a modification of haem for cytochrome c biogenesis, a complex process that entails both transport between cellular compartments and a specific thioether linkage between the haem moiety and the apoprotein. Finally, a whole genome analysis from 63 prokaryotes indicates intriguing exceptions to the universality of the haem biosynthetic pathway and helps define new frontiers for future study.

Structure and organization of a 25 kbp region of the genome of the photosynthetic green sulfur bacterium Chlorobium vibrioforme containing Mg-chelatase encoding genes

A region comprising approximately 25 kbp of the genome of the strictly anaerobic and obligate photosynthetic green sulfur bacterium Chlorobium vibrioforme has been mapped, subcloned and partly sequenced. Approximately 15 kbp have been sequenced in it's entirety and three genes with significant homology and feature similarity to the bchI, -D and -H genes and the chlI, -D and -H genes of Rhodobacter and Synechocystis strain PCC6803, respectively, which encode magnesium chelatase subunits, have been identified. Magnesium chelatase catalyzes the insertion of Mg2+ into protoporphyrin IX, and is the first enzyme unique to the (bacterio)chlorophyll specific branch of the porphyrin biosynthetic pathway. The organization of the three Mg-chelatase encoding genes is unique to Chlorobium and suggests that the magnesium chelatase of C. vibrioforme is encoded by a single operon. The analyzed 25 kbp region contains five additional open reading frames, two of which display significant homology and feature similarity to genes encoding lipoamide dehydrogenase and genes with function in purine synthesis, and another three display significant homology to open reading frames with unknown function in distantly related bacteria. Putative E. coli sigma 70-like promoter sequences, ribosome binding sequences and rho-independent transcriptional stop signals within the sequenced 15 kbp region are related to the identified genes and orfs. Southern analysis, restriction mapping and partial sequencing of the remaining ca. 10 kbp of the analyzed 25 kbp region have shown that this part includes the hemA, -C, -D and -B genes (MOBERG and AVISSAR 1994), which encode enzymes with function in the early part of the biosynthetic pathway of porphyrins.

Reconstitution of an active magnesium chelatase enzyme complex from the bchI, -D, and -H gene products of the green sulfur bacterium Chlorobium vibrioforme expressed in Escherichia coli

Magnesium-protoporphyrin chelatase, the first enzyme unique to the (bacterio)chlorophyll-specific branch of the porphyrin biosynthetic pathway, catalyzes the insertion of Mg2+ into protoporphyrin IX. Three genes, designated bchI, -D, and -H, from the strictly anaerobic and obligately phototrophic green sulfur bacterium Chlorobium vibrioforme show a significant level of homology to the magnesium chelatase-encoding genes bchI, -D, and -H and chlI, -D, and -H of Rhodobacter sphaeroides and Synechocystis strain PCC6803, respectively. These three genes were expressed in Escherichia coli; the subsequent purification of overproduced BchI and -H proteins on an Ni2+-agarose affinity column and denaturation of insoluble BchD protein in 6 M urea were required for reconstitution of Mg-chelatase activity in vitro. This work therefore establishes that the magnesium chelatase of C. vibrioforme is similar to the magnesium chelatases of the distantly related bacteria R. sphaeroides and Synechocystis strain PCC6803 with respect to number of subunits and ATP requirement. In addition, reconstitution of an active heterologous magnesium chelatase enzyme complex was obtained by combining the C. vibrioforme BchI and -D proteins and the Synechocystis strain PCC6803 ChlH protein. Furthermore, two versions, with respect to the N-terminal start of the bchI gene product, were expressed in E. coli, yielding ca. 38- and ca. 42-kDa versions of the BchI protein, both of which proved to be active. Western blot analysis of these proteins indicated that two forms of BchI, corresponding to the 38- and the 42-kDa expressed proteins, are also present in C. vibrioforme.