Inactivation of the antigen 85C gene profoundly affects the mycolate content and alters the permeability of the Mycobacterium tuberculosis cell envelope

The antigen 85 complex of Mycobacterium tuberculosis consists of three abun dantly secreted proteins. The recent characterization of a mycoloyltransfer ase activity associated in vitro with each of these antigens suggested that they are potentially important for the building of the unusual cell envelo pe of mycobacteria. To define the physiological role of these proteins, the gene coding for antigen 85C was inactivated by transposon mutagenesis. The resulting mutant was shown to transfer 40% fewer mycolates to the cell wal l with no change in the types of mycolates esterifying arabinogalactan or i n the composition of non-covalently linked mycolates. As a consequence, the diffusion of the hydrophobic chenodeoxycholate and the hydrophilic glycero l, but not that of isoniazid, was found to be much faster through the cell envelope of the mutant than that of the parent strain. Taken together, thes e data demonstrate that: (i) antigen 85C is involved directly or indirectly in the transfer of mycolates onto the cell wall of the whole bacterium; (i i) the enzyme is not specific for a given type of mycolate; and (iii) the c ell wall-linked mycolate layer may represent a barrier for the diffusion of small hydrophobic and hydrophilic molecules.