Isolation of cDNAs encoding GTP cyclohydrolase II from Arabidopsis thaliana

Biosynthesis of riboflavin in plants. The ribA gene of Arabidopsis thaliana specifies a bifunctional GTP cyclohydrolase II/3,4-dihydroxy-2-butanone 4-phosphate synthase

A cDNA segment from Arabidopsis thaliana with similarity to the ribA gene of Bacillus subtilis was sequenced. A similar gene was cloned from tomato. The open reading frame of A. thaliana was fused to the malE gene of Escherichia coli and was expressed in a recombinant E. coli strain. The recombinant fusion protein was purified and shown to have GTP cyclohydrolase II activity as well as 3,4-dihydroxy-2-butanone 4-phosphate synthase activity. The cognate gene was amplified by polymerase chain reaction from chromosomal Arabidopsis DNA and was shown to contain six introns. Intron 4 is located in the region connecting the GTP cyclohydrolase II and 3,4-dihydroxy-2-butanone 4-phosphate synthase domain of the putative domains catalyzing the two reaction steps. By comparison with the bacterial ribA gene, the Arabidopsis gene contains an additional 5' element specifying about 120 amino acid residues. This segment contains numerous serine and threonine residues and does not show similarity with other known sequences. The N-terminal segment is not required for catalytic activity and is likely to serve as signal sequence for import into chloroplasts.

Molecular analysis of riboflavin synthesis genes in Bartonella henselae and use of the ribC gene for differentiation of Bartonella species by PCR

The biosynthesis pathway for riboflavin (vitamin B(2)), the precursor of the essential cofactors flavin mononucleotide and flavin adenine dinucleotide, is present in bacteria and plants but is absent in vertebrates. Due to their conservation in bacterial species and their absence in humans, the riboflavin synthesis genes should be well suited either for detection of bacterial DNA in human specimens or for the differentiation of pathogenic bacteria by molecular techniques. A DNA fragment carrying the genes ribD, ribC, and ribE, which encode homologues of riboflavin deaminase (RibD) and subunits of riboflavin synthetase (RibC and RibE), respectively, was isolated from a plasmid-based DNA library of the human pathogen Bartonella henselae by complementation of a ribC mutation in Escherichia coli. Sequence analysis of the ribC gene region in strains of B. henselae, which were previously shown to be genetically different, revealed that the ribC gene is highly conserved at the species level. PCR amplification with primers derived from the ribC locus of B. henselae was used to isolate the corresponding DNA regions in B. bacilliformis, B. clarridgeiae, and B. quintana. Sequence analysis indicated that the riboflavin synthesis genes are conserved and show the same operon-like genetic organization in all four Bartonella species. Primer oligonucleotides designed on the basis of localized differences within the ribC DNA region were successfully used to develop species-specific PCR assays for the differentiation of B. henselae, B. clarridgeiae, B. quintana, and B. bacilliformis. The results obtained indicate that the riboflavin synthesis genes are excellent targets for PCR-directed differentiation of these emerging pathogens. The PCR assays developed should increase our diagnostic potential to differentiate Bartonella species, especially B. henselae and the newly recognized species B. clarridgeiae.

Plant riboflavin biosynthesis. Cloning, chloroplast localization, expression, purification, and partial characterization of spinach lumazine synthase

Lumazine synthase, which catalyzes the penultimate step of riboflavin biosynthesis, has been cloned from three higher plants (spinach, tobacco, and arabidopsis) through functional complementation of an Escherichia coli auxotroph. Whereas the three plant proteins exhibit some structural similarities to known microbial homologs, they uniquely possess N-terminal polypeptide extensions that resemble typical chloroplast transit peptides. In vitro protein import assays with intact chloroplasts and immunolocalization experiments verify that higher plant lumazine synthase is synthesized in the cytosol as a larger molecular weight precursor protein, which is post-translationally imported into chloroplasts where it is proteolytically cleaved to its mature size. The authentic spinach enzyme is estimated to constitute <0.02% of the total chloroplast protein. Recombinant "mature" spinach lumazine synthase is expressed in E. coli at levels exceeding 30% of the total soluble protein and is readily purified to homogeneity using a simple two-step procedure. Apparent V(max) and K(m) values obtained with the purified plant protein are similar to those reported for microbial lumazine synthases. Electron microscopy and hydrodynamic studies reveal that native plant lumazine synthase is a hollow capsid-like structure comprised of 60 identical 16.5-kDa subunits, resembling its icosahedral counterparts in E. coli and Bacillus subtilis.