The causative agents of leprosy and tuberculosis, Mycobacterium leprae and Mycobacterium tuberculosis, have a lipid-rich cell envelope which contributes to virulence and antibiotic resistance. Acyl coenzyme A carboxylase, which catalyzes the first committed step of lipid biosynthesis, consists in mycobacteria of two subunits, one of which is biotinylated. Genes from M. leprae and M. tuberculosis encoding a biotinylated protein have been cloned and sequenced. Analysis of the derived protein sequences demonstrated the presence of biotin-binding sites and putative ATP-bicarbonate interactions sites, consistent with the proteins having a biotin carboxylase function as well as their being biotin carrier proteins.