Gene switching and essentiality testing

Deletion of the Mycobacterium tuberculosis cyp138 gene leads to changes in membrane-related lipid composition and antibiotic susceptibility

Introduction

Mycobacterium tuberculosis (Mtb), the main cause of tuberculosis (TB), has brought a great burden to the world's public health. With the widespread use of Mtb drug-resistant strains, the pressure on anti-TB treatment is increasing. Anti-TB drugs with novel structures and targets are urgently needed. Previous studies have revealed a series of CYPs with important roles in the survival and metabolism of Mtb. However, there is little research on the structure and function of CYP138.

Methods

In our study, to discover the function and targetability of CYP138, a cyp138-knockout strain was built, and the function of CYP138 was speculated by the comparison between cyp138-knockout and wild-type strains through growth curves, growth status under different carbon sources, infection curves, SEM, MIC tests, quantitative proteomics, and lipidomics.

Results and discussion

The knockout of cyp138 was proven to affect the Mtb's macrophage infection, antibiotics susceptibility, and the levels of fatty acid metabolism, membrane-related proteins, and lipids such as triacylglycerol. We proposed that CYP138 plays an important role in the synthesis and decomposition of lipids related to the cell membrane structure as a new potential anti-tuberculosis drug target.

Structural and Genetic Analyses of the Mycobacterium tuberculosis Protein Kinase B Sensor Domain Identify a Potential Ligand-binding Site

Monitoring the environment with serine/threonine protein kinases is critical for growth and survival of Mycobacterium tuberculosis, a devastating human pathogen. Protein kinase B (PknB) is a transmembrane serine/threonine protein kinase that acts as an essential regulator of mycobacterial growth and division. The PknB extracellular domain (ECD) consists of four repeats homologous to penicillin-binding protein and serine/threonine kinase associated (PASTA) domains, and binds fragments of peptidoglycan. These properties suggest that PknB activity is modulated by ECD binding to peptidoglycan substructures, however, the molecular mechanisms underpinning PknB regulation remain unclear. In this study, we report structural and genetic characterization of the PknB ECD. We determined the crystal structures of overlapping ECD fragments at near atomic resolution, built a model of the full ECD, and discovered a region on the C-terminal PASTA domain that has the properties of a ligand-binding site. Hydrophobic interaction between this surface and a bound molecule of citrate was observed in a crystal structure. Our genetic analyses in M. tuberculosis showed that nonfunctional alleles were produced either by deletion of any of single PASTA domain or by mutation of individual conserved residues lining the putative ligand-binding surface of the C-terminal PASTA repeat. These results define two distinct structural features necessary for PknB signal transduction, a fully extended ECD and a conserved, membrane-distal putative ligand-binding site.

LytB1 and LytB2 of Mycobacterium tuberculosis Are Not Genetically Redundant

Mycobacterium tuberculosis synthesises isoprenoid precursors via the MEP/DOXP pathway and at least five enzymes in the pathway (Dxs1, Dxr/IspC, IspD, IspF, and GcpE/IspG) are required for growth in vitro. We investigated the role of LytB (IspH) in M. tuberculosis; M. tuberculosis is unusual in that it has two homologs–LytB1 and LytB2. We were unable to delete the lytB2 gene unless we provided an additional copy elsewhere, demonstrating that this is the essential homolog. We expressed lytB1 from the lytB2 promoter and confirmed that this could not complement for loss of function of lytB2, despite LytB1 possessing all the previously described conserved critical residues. Interestingly the sole LytB homolog of Mycobacterium smegmatis was able to compensate for loss of LytB2 in M. tuberculosis. We tested translational fusions of LytB1 and LytB2 for functionality in M. tuberculosis, but only a fusion with 90% N-terminal LytB2 and 10% C-terminal LytB1 was functional. In order to identify the key difference between the two proteins, site directed mutagenesis was used to change LytB2 residues into their counterparts in LytB1. None of these amino acid substitutions was essential for function and all lytB2 mutant alleles were functional. In contrast, mutation of the key residues for [Fe4S4] cluster formation, as well as a catalytic residue in LytB1 did not result in functional complementation. Thus, although LytB1 and LytB2 are not genetically redundant, this is not dependent on small amino acid changes, but is likely to be a result of major overall structural differences.

Targeted gene knockout and essentiality testing by homologous recombination

This chapter provides an updated experimental protocol for generating allelic exchange mutants of mycobacteria by two-step selection using the p2NIL/pGOAL system. The types of mutants that can be generated using this approach are targeted gene knockouts marked with a drug resistance gene, unmarked deletion mutants, or strains in which a point mutation/s has been introduced into the target gene. A method for assessing the essentiality of a gene for mycobacterial growth by means of allelic exchange is also described. This method, which utilizes a merodiploid strain carrying a second copy of the gene of interest on an integration vector, allows the exploration by means of complement switching of structure-function relationships in proteins that are essential for mycobacterial growth.