Lytic transglycosylases RlpA and MltC assist in Vibrio cholerae daughter cell separation

Abstract

The cell wall is a crucial structural feature in the vast majority of bacteria and comprises a rigid, covalently closed, mesh-like network of peptidoglycan (PG) strands. While PG synthesis is important for bacterial survival under many conditions, the cell wall is also a dynamic structure, undergoing degradation and remodeling by so-called \"autolysins\", enzymes that break bonds in the PG network. Cell division, for example, requires extensive PG remodeling and separation of daughter cells, which depends primarily upon the activity of amidases. However, in V. cholerae, we have found that amidase activity alone is insufficient for daughter cell separation and that the lytic transglycosylases RlpA and MltC both contribute to this process. MltC and RlpA both localize to the septum and are functionally redundant under normal laboratory conditions; however, only RlpA can support normal cell separation in low salt media. The division-specific activity of lytic transglycosylases has implications for the local structure of septal PG, suggesting that there may be glycan bridges between daughter cells that cannot be resolved by amidases. We propose that lytic transglycosylases at the septum serve as a back-up mechanism to cleave rare, stochastically produced PG strands that are crosslinked beyond the reach of the highly spatio-temporally limited activity of the amidase and to clear PG debris that may block the completion of outer-membrane invagination.