Identification and characterization of two novel methyltransferase genes that determine the serotype 12-specific structure of glycopeptidolipids of Mycobacterium intracellulare.

The Mycobacterium avium complex is distributed ubiquitously in the environment. It is an important cause of pulmonary and extrapulmonary diseases in humans and animals. The species in this complex produce polar glycopeptidolipids (GPLs); of particular interest is their serotype-specific antigenicity. Several reports have described that GPL structure may play an important role in bacterial physiology and pathogenesis and in the host immune response. Recently, we determined the complete structure of the GPL derived from Mycobacterium intracellulare serotype 7 and characterized the serotype 7 GPL-specific gene cluster. The structure of serotype 7 GPL closely resembles that of serotype 12 GPL, except for O methylation. In the present study, we isolated and characterized the serotype 12-specific gene cluster involved in glycosylation of the GPL. Ten open reading frames (ORFs) and one pseudogene were observed in the cluster. The genetic organization of the serotype 12-specific gene cluster resembles that of the serotype 7-specific gene cluster, but two novel ORFs (orfA and orfB) encoding putative methyltransferases are present in the cluster. Functional analyses revealed that orfA and orfB encode methyltransferases that synthesize O-methyl groups at the C-4 position in the rhamnose residue next to the terminal hexose and at the C-3 position in the terminal hexose, respectively. Our results show that these two methyltransferase genes determine the structural difference of serotype 12-specific GPL from serotype 7-specific GPL.