PEPTIDOGLYCAN REMODELLING FACTORS: KEY PLAYERS IN CELL DIVISION AND HOMEOSTASIS MAINTENANCE OF ESCHERICHIA COLI

In Escherichia coli, the structural integrity and coordinated growth of the cell envelope are crucial for survival, especially under environmental stress conditions. The lipopolysaccharide-containing outer membrane (OM) acts as a selective barrier against toxic molecules and provides additional mechanical support that complements the role traditionally attributed to the cell wall. Coordination between the OM and peptidoglycan (PG) growth is essential for maintaining cell envelope stability, particularly during cell division, when septal PG must be cleaved to allow daughter cell separation. This process is mediated by PG amidases, AmiA, AmiB, and AmiC, that are activated by LytM-domain proteins both from inside the cell, with the periplasmic protein EnvC that is anchored to the divisome components FtsEX, and the outside of the cell thanks to the OM lipoproteins NlpD and ActS. In the first manuscript included in this thesis, it is shown the identification of the periplasmic deacetylase SddA, responsible for modifying the amidase cleavage products. The expression of the non-essential sddA gene is linked with that of EnvC, highlighting the involvement of SddA in cell separation. Consistently with this speculation, the overexpression of sddA leads to the formation of long chains of unseparated cells, indicating that amidase activity at the septum is inhibited. The amidase activator ActS also displays a central role in envelope stress conditions. It has been shown that when OM biogenesis is disrupted, DpaA, the enzyme that detaches the OM lipoprotein Lpp from PG, is critical for preventing cell lysis. The lysis phenotype of dpaA deleted cells with defective LPS export is suppressed by the deletion of actS, suggesting that under LPS stress DpaA is capable of modulating ActS activity. A second manuscript included in this thesis shows that a DpaA-mediated modulation occurs at the ActS protein level. Together, these findings reveal a complex regulatory mechanism linking OM and PG growth, septal PG cleavage, and cell envelope remodeling under stress. This coordination ensures structural stability and viability of E. coli by integrating responses to envelope stress with the mechanical demands of cell division.