The β-lactam resistance protein Blr, a small membrane polypeptide, is a component of the Escherichia coli cell division machinery.

In Escherichia coli, cell division is performed by a multimolecular machinery called the divisome, made of 10 essential proteins and more than 20 accessory proteins. Through a bacterial two-hybrid library screen, we identified the E. coli β-lactam resistance protein Blr, a short membrane polypeptide of 41 residues, as an interacting partner of the essential cell division protein FtsL. In addition to FtsL, Blr was found to associate with several other divisomal proteins, including FtsI, FtsK, FtsN, FtsQ, FtsW, and YmgF. Using fluorescently tagged Blr, we showed that this peptide localizes to the division septum and that its colocalization requires the presence of the late division protein FtsN. Although Blr is not essential, previous studies have shown that the inactivation of the blr gene increased the sensitivity of bacteria to β-lactam antibiotics or their resistance to cell envelope stress. Here, we found that Blr, when overproduced, restores the viability of E. coli ftsQ1(Ts) cells, carrying a thermosensitive allele of the ftsQ gene, during growth under low-osmotic-strength conditions (e.g., in synthetic media or in Luria-Bertani broth without NaCl). In contrast, the inactivation of blr increases the osmosensitivity of ftsQ1(Ts) cells, and blr ftsQ1 double mutants exhibit filamentous growth in LB broth even at a moderate salt concentration (0.5% NaCl) compared to parental ftsQ1(Ts) cells. Altogether, our results suggest that the small membrane polypeptide Blr is a novel component of the E. coli cell division apparatus involved in the stabilization of the divisome under certain stress conditions.