Analysis of the Phthiocerol Dimycocerosate Locus ofMycobacterium tuberculosis

Abstract

Among the few characterized genes that have prod- ucts involved in the pathogenicity of Mycobacterium tu- berculosis, the etiological agent of tuberculosis, are those of the phthiocerol dimycocerosate (DIM) locus. Genes involved in biosynthesis of these compounds are grouped on a 50-kilobase fragment of the chromosome containing 13 genes. Analysis of mRNA produced from this 50-kilobase fragment in the wild type strain showed that this region is subdivided into three transcriptional units. Biochemical characterization of five mutants with transposon insertions in this region demonstrated that (i) the complete DIM molecules are synthesized in the cytoplasm of M. tuberculosis before being translo- cated into the cell wall; (ii) the genes fadD26 and fadD28 are directly involved in their biosynthesis; and (iii) both the drrC and mmpL7 genes are necessary for the proper localization of DIMs. Insertional mutants unable to syn- thesize or translocate DIMs exhibit higher cell wall per- meability and are more sensitive to detergent than the wild type strain, indicating for the first time that, in addition to being important virulence factors, extracta- ble lipids of M. tuberculosis play a role in the cell enve- lope architecture and permeability. This function may represent one of the molecular mechanisms by which DIMs are involved in the virulence of M. tuberculosis. Mycobacterium