Discovery of Analogous Enzymes in Thiamin Biosynthesis by Anticorrelation

Abstract

Prediction of gene function is one of the most challenging tasks in genomic science when there is noclear sequence similarity to annotated genes. In all genome-sequencing projects completed to date, aconsiderable amount of \gaps have been found in the biochemical pathways of the respective species.In many instances, missing enzymes are displaced by analogs: functionally equivalent proteins thathave evolved independently and lack sequence and structural similarity. We analysed the thiamin(vitamin B1) biosynthesis pathway in the completely sequenced microorganisms. We used two ap-proaches: rst, genomic context and the second, anticorrelation of occurrence of genes in the genomes.Using these criteria we identi ed several di erent ORFs which do not have homology to any knownthi gene from Escherichia coli which might be involved in thiamin biosynthesis. We have cloned someof these genes from Thermotoga maritima, Saccharomyces cerevisiae, Bacillus subtilis, and Rhizo-bium etli and used them to complement the thiamin de ciency of thiC, thiE, thiG and thiH mutantstrain generated by precise gene deletion. Our results demonstrated that seven of these genes indeedparticipate in thiamin biosynthesis and shown that gene displacement has occurred often in thiaminbiosynthesis.