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.