Discovery of Analogous Enzymes in Thiamin Biosynthesis by Anticorreiation

摘要

Prediction of gene function is one of the most challenging tasks in genomic science when there is no clear sequence similarity to annotated genes. In all genome-sequencing projects completed to date, a considerable amount of "gaps" have been found inthe biochemical pathways of the respective species. In many instances, missing enzymes are displaced by analogs: functionally equivalent proteins that have evolved independently and lack sequence and structural similarity. We analysed the thiamin (vitamin B1) biosynthesis pathway in the completely sequenced microorganisms. We used two approaches: first, genomic context and the second, anticorrelation of occurrence of genes in the genomes. Using these criteria we identified several different ORFs which do not have homology to any known thi gene from Escherichia coli which might be involved in thiamin biosynthesis. We have cloned some of these genes from Thermotoga maritime, Saccharomyces cerevisiae, Bacillus subtilis, and Rhizobium etli and used them tocomplement the thiamin deficiency of thiC, thiE, thiG and thiH mutant strain generated by precise gene deletion. Our results demonstrated that seven of these genes indeed participate in thiamin biosynthesis and shown that gene displacement has occurredoften in thiamin biosynthesis.