The impact of mab_1638 on the M. abscessus transcriptome
Mycobacterium abscessus is a major human pathogen, mostly infecting people with pre-existing pathological lung conditions such as cystic fibrosis. Only little is known about the ways M. abscessus regulates its virulence factors and pathogenesis phenotypes. The production of glycopeptidolipids (GPL) is a major determinant of virulence of this bacterium, with clinical isolates that lack GPL synthesis and export generally exhibiting more aggressive clinical behavior. The current paradigm is that GPL production is abolished in vivo via irreversible, spontaneous mutations taking place as part of in-host evolution. Little is known about the mechanisms or extent to which GPL production may be regulated. We have found an unusual TetR-like transcription factor of M. abscessus, mab_1638, identified via screening of a comprehensive transposon-mutant library, that appears to be a strong positive regulator of the entire GPL biosynthesis and export gene cluster through a combination of direct and indirect mechanisms. The inactivation of mab_1638 by a transposon led to stable and visually obvious rough colony morphology, as well as increased virulence in infection models, characteristic of rough, non-GPL-producers. Transcriptome analysis found the mab_1638:tn mutant to have 118 differentially expressed genes, including the GPL locus. It appears mab_1638 encodes an unusual TetR transcription factor required for  the production of GPL in M. abscessus and therefore having a profound effect on virulence traits. This finding raises the important possibility that M. abscessus strains appearing smooth in laboratory growth conditions may nonetheless downregulate GPL-cluster genes in other conditions and thus acquire the phenotypic characteristics of rough strains. Overall design: The mab_1638 gene of M. abscessus strain ATCC19977 was disrupted with a transposon. Triplicate cultures of the parental and mab_1638::tn strains were grown to log phase and RNA harvested and used to contruct RNAseq expression libraries.