Functional Complementation of the Essential Gene fabG1 of Mycobacterium tuberculosis by Mycobacterium smegmatis fabG but Not Escherichia coli fabG

Mycolic acids are a key component of the mycobacterial cell wall, providing structure and forming a major permeability barrier. InMycobacterium tuberculosis mycolic acids are synthesized by type I and type II fatty acid synthases. One of the enzymes of the type II system is encoded byfabG1 . We demonstrate here that this gene can be deleted from theM. tuberculosis chromosome only when another functional copy is provided elsewhere, showing that under normal culture conditionsfabG1 is essential. FabG1 activity can be replaced by the corresponding enzyme from the closely related speciesMycobacterium smegmatis but not by the enzyme fromEscherichia coli. M. tuberculosis carrying FabG fromM. smegmatis showed no phenotypic changes, and both the mycolic acids and cell wall permeability were unchanged. Thus,M. tuberculosis andM. smegmatis enzymes are interchangeable and do not control the lengths and types of mycolic acids synthesized.