AN EMBRYO-DEFECTIVE MUTANT OF ARABIDOPSIS DISRUPTED IN THE FINAL STEPOF BIOTIN SYNTHESIS

Auxotrophic mutants have played an important role in the genetic disse ction of biosynthetic pathways in microorganisms. Equivalent mutants h ave been more difficult to identify in plants. The biol auxotroph of A rabidopsis thaliana was shown previously to be defective in the synthe sis of the biotin precursor 7,8-diaminopelargonic acid. A second bioti n auxotroph of A. thaliana has now been identified. Arrested embryos f rom this bio2 mutant are defective in the final step of biotin synthes is, the conversion of dethiobiotin to biotin. This enzymatic reaction, catalyzed by the bioB product (biotin synthase) in Escherichia coli, has been studied extensively in plants and bacteria because it involve s the unusual addition of sulfur to form a thiophene ring. Three lines of evidence indicate that bio2 is defective in biotin synthase produc tion: mutant embryos are rescued by biotin but not dethiobiotin, the m utant allele maps to the same chromosomal location as the cloned bioti n synthase gene, and gel-blot hybridizations and polymerase chain reac tion amplifications revealed that homozygous mutant plants contain a d eletion spanning the entire BIO2-coding region. Here we describe how t he isolation and characterization of this null allele have provided va luable insights into biotin synthesis, auxotrophy, and gene redundancy in plants.