Resurgence of tuberculosis (TB) and other mycobacterial infections associated with AIDS threaten human health world-wide. Emergence of drug resistant TB makes it critical to discover new drug targets. Cell walls of Mycobacterium tuberculosis contain a variety of unusual lipids with many types of multiple methyl-branched fatty acids that are unique to pathogenic mycobacteria. These lipids are thought to play important roles in the ability of the pathogens to resist antimicrobial agents and evade the defense reactions of the host. Biosynthesis of these unique lipids containing multiple methyl-branched fatty acids could offer ideal targets for new anti mycobacterial drugs. Genomic sequencing revealed that a remarkably unique feature of the mycobacterial genome is that it contains an unusually large number of genes involved in lipid metabolism. Based on the homology to mycocerosic acid synthase (mas) we have identified two classes of polyketide synthase (pks)-like genes which contain a full complement of active site domains that should be involved in the catalysis of all of the steps required for the synthesis of multimethyl- branched fatty acids: Class 1 mas-like genes mas-like genes (msl1, msl2 and msl3) that are highly homologous to mas and Class 2 (msl4m msl5, msl6 and msl7) which show a lesser degree of homology to mas. These open reading frames (ORFs) most probably encode the more than eight classes of methyl-branched fatty acids found in M. tuberculosis. To elucidate the function of these genes and to examine their possible role in the host-pathogen interaction, we propose to pursue the following specific aims: 1) Determine the functions of Class 1 mas-like genes, ms1, msl2 and msl3 of M. tuberculosis by characterization of their products expressed in M. smegmatis, and by determination of the biochemical and functional consequences of their disruption. 2) Determine the functions of Class 2 (msl4, msl5, msl6 and msl7) mas-like genes in M. tuberculosis genome, by characterization of their products expressed in M. smegmatis, and by determination of the biochemical and functional consequences of their disruption. 3) Determine whether lack of specific lipids caused by the above gene-disruptions affects host-pathogen interaction and virulence. The results will help identify cell wall lipids critical for pathogenesis that might be suitable targets for new anti-mycobacterial drugs.