MurJ and a novel lipid II flippase are required for cell wall biogenesis in Bacillus subtilis

Area of Science:

• Microbiology
• Molecular Biology
• Biochemistry

Background:

• Bacterial cell wall polysaccharides are synthesized via lipid-linked precursors.
• Transport of lipid II, a cell wall precursor, across the cytoplasmic membrane is crucial for bacterial envelope assembly.
• In Escherichia coli, this transport is primarily mediated by MurJ, an essential MOP exporter superfamily member.

Purpose of the Study:

• To identify alternative lipid II flippases in bacteria, as Bacillus subtilis lacking all MOP superfamily members remains viable.
• To investigate the function of the uncharacterized gene amj and its relationship with B. subtilis MurJ (MurJBs).

Main Methods:

• Transposon sequencing was employed to screen for synthetic lethal partners of MOP family members.
• Functional analysis of Amj and MurJBs in B. subtilis and E. coli.
• Gene expression analysis under the control of the sigma factor σ(M).

Main Results:

• Bacillus subtilis cells lacking all 10 MOP superfamily members show only minor morphological defects, indicating an alternative lipid II flippase.
• The uncharacterized gene amj and B. subtilis MurJ (MurJBs) form a synthetic lethal pair.
• Amj and MurJBs can functionally complement E. coli MurJ, supporting lipid II flipping and cell viability.
• Amj represents a novel family of flippases found in various bacteria.
• Amj expression is regulated by the cell envelope stress response factor σ(M), and its transcription increases in MurJBs-deficient cells.

Conclusions:

• A novel lipid II flippase, Amj, exists in bacteria, providing an alternative to the essential MurJ.
• Amj and MurJBs function redundantly, and their simultaneous absence leads to cell lysis.
• The discovery of Amj opens possibilities for targeting bacterial cell wall synthesis, potentially by inhibiting MurJ and inducing Amj expression.