LOCI OF MYCOBACTERIUM-AVIUM SER2 GENE-CLUSTER AND THEIR FUNCTIONS

The highly antigenic glycopeptidolipids present on the surface of memb ers of the Mycobacterium avium complex serve to distinguish these bact eria from all others and to define the various serovars that compose t his complex. Previously, the genes responsible for the biosynthesis of the disaccharide hapten [2,3-di-O-methyl- ha-L-fucopyranosyl-(1-->3)- alpha-L-rhamnopyranose] of serovar 2 of the M. avium complex were isol ated, localized to a contiguous 22- to 27-kb fragment of the M. avium genome, and designated the ser2 gene cluster (J. T. Belisle, L. Pascop ella, J. M. Inamine, P. J. Brennan, and W. R. Jacobs, Jr., J. Bacterio l. 173:6991-6997; 1991). In the present study, transposon saturation m utagenesis was used to map the specific genetic loci within the ser2 g ene cluster required for expression of this disaccharide. Four essenti al loci, termed ser2A, -B, -C, and -D, constituting a total of 5.7 kb within the ser2 gene cluster, were defined. The ser2B and ser2D loci e ncode the methyltransferases required to methylate the fucose at the 3 and 2 positions, respectively. The rhamnosyltransferase was encoded b y ser2A, whereas either ser2C or ser2D encoded the fucosyltransferase. The ser2C and ser2D loci are also apparently involved in the de novo synthesis of fucose. Isolation of the truncated versions of the hapten induced by the transposon insertions provides genetic evidence that t he glycopeptidolipids of M. avium serovar 2 are synthesized by an init ial transfer of the rhamnose unit to the peptide core followed by fuco se and finally O methylation of the fucosyl unit.