A Francisella tularensis L,D-carboxypeptidase plays important roles in cell morphology, envelope integrity, and virulence

Abstract

word count: 192 21 Text word count: 9082 22 Abstract 23 F. tularensis is a Gram-negative, intracellular bacterium that causes the zoonotic disease 24 tularemia. Intracellular pathogens, including F. tularensis, have evolved mechanisms to allow 25survival in the harsh environment of macrophages and neutrophils, where they are exposed to 26 cell membrane-damaging molecules. One mechanism that protects intracellular Gram-negative 27 bacteria from macrophage or neutrophil killing is the ability to recycle and repair damaged 28 peptidoglycan (PG),a process that requires over 50 different PG synthesis and recycling 29 enzymes. In addition to PG repair, PG recycling occurs during cell division and plays critical 30 roles in maintaining cell morphology, structure, and membrane integrity. Here, we identified a 31 PG recycling enzyme, L,D-carboxypeptidase A (LdcA), of F. tularensis that is responsible for 32 converting PG tetrapeptide stems to tripeptide stems. Unlike prototypic LdcA homologs, F. 33 tularensis LdcA is outer membrane-associated and also exhibits L,D-endopeptidase activity, 34 converting PG pentapeptide stems to tripeptide stems. Loss of F. tularensis LdcA led to altered 35 cell morphology and membrane integrity, as well as attenuation in a mouse pulmonary infection 36 model and in primary and immortalized macrophages. Finally, a F. tularensis LdcA mutant 37 protected mice against virulent Type A F. tularensis SchuS4 pulmonary challenge. 38 39 Importance: PG is well known to play important roles in protecting bacteria from external 40 insults and osmotic stress. However, pathogenic bacteria modify their PG architecture to avoid 41 antibiotic activity and/or encode PG recycling and repair pathways to promote bacterial 42 virulence. Prototypic Gram-negative bacteria, including E. coli and Pseudomonas aeruginosa, 43 recycle PG in a multi-enzyme pathway, including the cytoplasmic L,D-carboxypeptidase, LdcA.44Here, we demonstrated that a previously unstudied protein from the intracellular bacterium F. tularensis contains an LdcA conserved domain, this F. tularensis LdcA is outer membrane-46 associated, and deletion of this LdcA ortholog increases bacterial outer membrane thickness, 47 resulting in bacteria that are more resistant to various stressors (e.g., H 2 O 2 , NaCl, low pH), yet 48 are completely attenuated in a mouse pulmonary infection model. Finally, we demonstrated that 49 the F. tularensis LdcA mutant can be used as a live, attenuated vaccine to protect against 50 pulmonary challenge with fully virulent F. tularensis. 51 52 53 54 The Gram-negative bacterial cell wall plays an important role in maintaining cell shape, 55 protecting against external insults, and preventing cell lysis amid fluctuations in internal turgor 56 pressure (1). The cell wall is composed of peptidoglycan (PG), a network of alternating N-57 acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc) glycan chains that are 58 crosslinked through peptide stems, and lies just outside of the cytoplasmic membrane of most 59 bac...