THE LINK BETWEEN THE FAS SYSTEMS IN MYCOBACTERIA: A MISSING PIECE IN THE MODEL OF FATTY ACIDS BIOSYNTHESIS

SAVORETTI F, , ; CROTTAASIS A; GRAMAJO H,; GAGO G

Congreso; SAIB 2020; 2020

THE LINK BETWEEN THE FAS SYSTEMS IN MYCOBACTERIA: A MISSING PIECE IN THE MODEL OF FATTY ACIDS BIOSYNTHESISSavoretti F, CrottaAsis A, Gramajo H, Gago GInstituto de Biología Molecular y Celular de Rosario, Rosario, Argentina. E-mail: savoretti@ibr-conicet.gov.arMycobacterium tuberculosis has a very complex life style. The flexibility in its metabolism allows it to adapt and survive within the infected host. During this process, one of the most affected pathways is lipid metabolism, both in the host and in the pathogen. Despite there is a lot of information about the biosynthesis, structure and biological function of the main lipids present in M. tuberculosis envelope, little is known about the mechanisms that allow the bacteria modulate and adapt the biosynthesis of the components of the cell wall in response to changes in environment. Thus, the study of the processes involved in the regulation of the biosynthesis of lipids in M. tuberculosis represents a crucial step in the comprehension of the physiology and physiopathology of this pathogen, as well as to understand the interaction between mycobacteria and the environment. The biosynthesis of fatty acids in M. tuberculosis involves two different systems of fatty acid synthases (FAS I and FAS II). Both synthases are involved in the biosynthesis of membrane fatty acids and several lipid components of the cell wall, like mycolic acids which are essential for viability and pathogenesis. Therefore, they have to work in a coordinate way to keep lipid homeostasis. These two systems are linked by a beta-ketoacyl-acyl carrier protein synthase III, named FabH, that catalyzes a condensing reaction combining acyl-CoAs produced by FAS I with malonyl-ACP to form beta-ketoacyl-ACP. This product is the substrate of the FAS II system and is elongated to produce the precursors of the mycolic acids. Although FabH has been studied at the biochemical level, genetic analysis is needed in order to unequivocally establish the physiological role of this enzyme. In this work we performed a homology sequence search over different Mycobacterium genome database focusing on lipid biosynthesis, and we found out that while the fas gene encoding FAS-I and the genes encoding the FAS-II system (acpM, fabD, kasA, kasB, hadABC, mabAand inhA) are highly conserved in mycobacteria, the gene fabH it is not. For example, it is not present in Mycobacterium leprae and Mycobacterium fortuitum. Additionally, several Transposon Site Hybridization (TraSH) experiments demonstrated that fabH is not essential for M. tuberculosis survival. Considering that FabH has a central role in the current model of lipid metabolism, these results indicate that there should be another protein or pathway implicated in the link of the FAS systems. In order to unravel this, we obtained a knockout mutant in the single putative gene for fabH in Mycobacterium smegmatis and demonstrated that it is not essential for in vitro growth. Moreover, mycolic acids biosynthesis was not altered in the mutant in the conditions studied. Altogether, our analysis suggests that there is a missing piece in the model of synthesis of fatty acids in mycobacteria