Structural effects of mutations in Salmonella typhimurium flagellar switch complex

Area of Science:

• Microbiology
• Molecular Biology
• Structural Biology

Background:

• Salmonella typhimurium flagellar proteins FliG, FliM, and FliN are essential for motility and chemotaxis.
• These proteins are thought to form a complex within the flagellar basal body.
• Mutations in these genes result in non-motile or non-chemotactic bacterial phenotypes.

Purpose of the Study:

• To investigate the structural roles of FliG and FliM in Salmonella flagellar basal bodies.
• To elucidate the protein-protein interactions between FliG and FliM.
• To understand the structural differences between non-motile and non-chemotactic Salmonella mutants.

Main Methods:

• Electron microscopy of flagellar basal body preparations from mutant Salmonella strains.
• Immunoblot gel analysis using antibodies against FliG and FliM.
• Analysis of flagellar structures from wild-type and mutant bacteria, including temperature-sensitive mutants.

Main Results:

• Non-motile mutants often lacked FliM but contained FliG, with altered basal body morphology.
• Non-motile mutants with depleted FliG but present FliM showed extended, belled structures labeled with anti-FliM.
• FliG and FliM demonstrated interaction, as FliM was lost with FliG in temperature-sensitive mutants.
• Non-chemotactic mutants exhibited flagellar basal structures indistinguishable from wild-type.

Conclusions:

• FliM is a component of the outer shell of the extended flagellar basal structure, contributing to its belled morphology.
• FliG is likely part of the inner substructure of the flagellar basal body.
• The distinct structural integrity of flagellar basal bodies in non-motile versus non-chemotactic mutants suggests differences in protein interactions during torque generation and rotation switching.