CHARACTERIZATION OF THE CDNA AND GENE CODING FOR THE BIOTIN SYNTHASE OF ARABIDOPSIS-THALIANA

Biotin, an essential cofactor, is synthesized de novo only by plants a nd some microbes. An Arabidopsis thaliana expressed sequence tag that shows sequence similarity to the carboxyl end of biotin synthase from Escherichia coli was used to isolate a near-full-length cDNA. This cDN A was shown to code for the Arabidopsis biotin synthase by its ability to complement a bioB mutant of E. coli. Site-specific mutagenesis ind icates that residue threonine-173, which is highly conserved in biotin synthases, is important for catalytic competence of the enzyme. The p rimary sequence of the Arabidopsis biotin synthase is most similar to biotin synthases from E. coli, Serratia marcescens, and Saccharomyces cerevisiae (about 50% sequence identity) and more distantly related to the Bacillus sphaericus enzyme (33% sequence identity). The primary s equence of the amino terminus of the Arabidopsis biotin synthase may r epresent an organelle-targeting transit peptide. The single Arabidopsi s gene coding for biotin synthase, BIO2, was isolated and sequenced. T he biotin synthase coding sequence is interrupted by five introns. The gene sequence upstream of the translation start site has several unus ual features, including imperfect palindromes and polypyrimidine seque nces, which may function in the transcriptional regulation of the BIO2 gene.