Peptidoglycan synthesis in Mycobacterium tuberculosis is organized into networks with varying drug susceptibility Karen J. Kiesera, Catherine Baranowskia, Michael C. Chaob, Jarukit E. Longc, Christopher M. Sassettic,d, Matthew K. Waldorb,d,e, James C. Sacchettinif, Thomas R. Ioergerg, and Eric J. Rubina,e,1 aDepartment of Immunology and Infectious Disease, Harvard T. H. Chan School of Public Health, Boston, MA 02115; bDivision of Infectious Diseases, Brigham & Women’s Hospital, Boston, MA 02115; cDepartment of Microbiology and Physiological Systems, University of Massachusetts Medical School, Worcester MA 01655; dHoward Hughes Medical Institute, Chevy Chase, MD 20815; eDepartment of Microbiology and Immunobiology, Harvard Medical School, Boston, MA 02115; fDepartment of Biochemistry and Biophysics and Department of Chemistry, Texas A&M University, College Station, TX 77843; and gDepartment of Computer Science, Texas A&M University, College Station, TX 77843 Edited by Carolyn R. Bertozzi, University of California, Berkeley, CA, and approved September 8, 2015 (received for review July 17, 2015) Peptidoglycan (PG), a complex polymer composed of saccharide proper cell-wall synthesis. One method of coordination is the use chains cross-linked by short peptides, is a critical component of the of large protein complexes, the elongation complex and divisome, bacterial cell wall. PG synthesis has been extensively studied in which mediate cell-wall biogenesis during cell elongation or di- model organisms but remains poorly understood in mycobacteria, vision, respectively (2). The essential activity of these enzymes a genus that includes the important human pathogen Mycobacte- makes them prime drug targets; indeed, PBPs and Ldts are inhibited rium tuberculosis (Mtb). The principle PG synthetic enzymes have by carbapenems and penicillin (6, 7), which remains one of the most similar and, at times, overlapping functions. To determine how these clinically important drugs in use. are functionally organized, we carried out whole-genome transposon Although the biosynthesis and structure of PG have been in- mutagenesis screens in Mtb strains deleted for ponA1, ponA2,and vestigated for decades, predominantly in organisms such as ldtB, major PG synthetic enzymes. We identified distinct factors Escherichia coli or Bacillus subtilis, the mechanisms that coordinate the biochemical activities required to polymerize and modify the required to sustain bacterial growth in the absence of each of these MICROBIOLOGY cell wall remain incompletely understood. Moreover, much less enzymes. We find that even the homologs PonA1 and PonA2 have is known about PG synthesis in many pathogenic organisms, unique sets of genetic interactions, suggesting there are distinct PG including Mtb (2). However, previous studies in Mtb suggest that synthesis pathways in Mtb. Either PonA1 or PonA2 is required for Mtb PG synthesis in this pathogen does not strictly conform to the growth of , but both genetically interact with LdtB, which has E. coli paradigm. For example, E. coli has three bifunctional its own distinct genetic network. We further provide evidence that PBPs [PBP1A, PBP1B, and PBP1C (3)], whereas Mtb has just each interaction network is differentially susceptible to antibiotics. two [PonA1 and PonA2 (8)]. Additionally, PBP2 (known as Thus, Mtb uses alternative pathways to produce PG, each with its PBPA in mycobacteria) is a monofunctional PBP and is required own biochemical characteristics and vulnerabilities. for cell elongation in E. coli, but instead seems to function in cell septation in mycobacteria (3, 9). Mycobacterium tuberculosis | transposon sequencing | The structure of PG is also different in Mtb than in E. coli: genetic interaction | peptidoglycan | cell wall Mycobacterial PG has an unusual prevalence of 3–3 peptide linkages. The abundance of 3–3 cross-links in mycobacterial PG ne of the leading causes of infectious disease deaths world- throughout different growth stages (5) suggests that Ldts are Owide is tuberculosis (TB), caused by Mycobacterium tubercu- losis (Mtb). One-third of the human population is thought to Significance harbor Mtb and ∼1.5 million individuals died of TB last year (1). Mtb’s success as a pathogen is due in part to its unusual cell wall, The rise of drug-resistant Mycobacterium tuberculosis (Mtb) which is notorious for its complexity and is implicated in Mtb’s underscores the critical need for a better understanding of es- innate resistance to many commonly used antibiotics (2). A sential physiological processes. Among these is cell-wall syn- critical component of the bacterial cell wall (including Mtb’s) is thesis, the target of many antibiotics. To understand how Mtb peptidoglycan (PG), a complex polymer that provides structural orchestrates synthesis of its cell wall, we performed whole- support and counteracts turgor pressure (3). PG is essential for genome interaction studies in cells with different peptidoglycan cell survival, and its synthesis is targeted by many potent anti- synthesis mutations. We found that different enzymes become biotics (2). required for bacterial growth in ΔponA1, ΔponA2,orΔldtB cells, PG consists of long glycan chains composed of two different suggesting that discrete cell envelope biogenesis networks exist sugars (Fig. 1A) that are cross-linked via short peptide side in Mtb. Furthermore, we show that these networks’ enzymes chains that extend from the glycan chains. Notably, generation of are differentially susceptible to cell-wall–active drugs. Our data mature PG occurs outside of the cell membrane and is mediated provide insight into the essential processes of cell-wall synthesis by enzymes that incorporate new PG subunits, which are formed in Mtb and highlight the role of different synthesis networks in in the cytoplasm, into the PG polymer. PonA1 and PonA2 are antibiotic tolerance. the two enzymes in Mtb that can both polymerize glycan strands and cross-link peptides [known as bifunctional penicillin binding Author contributions: K.J.K., C.B., and E.J.R. designed research; K.J.K., C.B., and J.E.L. proteins (PBPs), Fig. 1A]. The predominant peptide cross-links performed research; K.J.K., C.B., M.C.C., J.E.L., C.M.S., M.K.W., J.C.S., and T.R.I. contributed in mycobacteria join the third amino acids (3–3 link) of adjacent new reagents/analytic tools; K.J.K., C.B., M.C.C., T.R.I., and E.J.R. analyzed data; and K.J.K. and E.J.R. wrote the paper. stem peptides (4, 5), which are synthesized by L,D-transpeptidases (Ldts) such as LdtB, one of the major Ldts in Mtb (Fig. 1A). The The authors declare no conflict of interest. peptides can also be joined by cross-linking the fourth and third This article is a PNAS Direct Submission. amino acids (4–3link)(Fig.1A) through the action of bifunctional 1To whom correspondence should be addressed. Email: [email protected]. or monofunctional (capable of only peptide cross-linking) PBPs. This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. The activity of these distinct factors must be coordinated to ensure 1073/pnas.1514135112/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1514135112 PNAS Early Edition | 1of6 Downloaded by guest on October 2, 2021 Here, we interrogated PG synthesis in Mtb by investigating the genetic interactions of ponA1, ponA2, and ldtB, which encode three PG synthases critical for Mtb’s growth during infection. To identify these interactions, we performed genome-wide transposon mutagenesis screens in Mtb mutant strains that lacked one of these enzymes. Advances in high-throughput sequencing technology coupled with the power of whole-genome studies provide unique insights into key bacterial processes, such as cell-wall biosynthesis. Such studies have been performed to a limited extent in bacteria, and further work would substantially expand our understanding of the organization of prokaryotic metabolic processes. In this study, we identified diverse genetic interaction networks for ponA1, ponA2, and ldtB, suggesting that these synthases are embedded within distinct cellular networks for assembling Mtb’s PG. We found that either ponA1 or ponA2 is required for cell growth, and that ldtB interacts with both ponA1 and ponA2. Moreover, mu- tants that lack these enzymes have differential susceptibility to agents that interfere with cell-wall biogenesis. Thus, the Mtb cell wall is synthesized using multiple interacting networks that are both overlapping and unique. Results PonA1, PonA2, and LdtB Are Individually Dispensable for Growth of Mtb. PonA1, its homolog PonA2, and LdtB can generate bonds between new PG subunits and those in the existing PG polymer (Fig. 1A). We generated independent ponA2 and ldtB deletion mutants and assessed their growth. As suggested by previous studies (14, 18) absence of either gene did not substantially affect Mtb’s exponential growth, although loss of LdtB diminished pop- ulation density in stationary phase (Fig. 1B). However, ΔponA2 cells were moderately wider than wild-type Mtb (Fig. 1C). In contrast, ΔponA1 cells are longer than wild-type cells (16), suggesting Fig. 1. Deletion of PG synthases influences growth and morphology of Mtb. these two homologous enzymes have distinct roles in PG synthesis (A) Transglycosylation (TG) and transpeptidation (TP) reactions incorporate in Mtb. Mutant cells that lack LdtB were significantly shorter [as new PG subunits into the cell wall. PonA1 and PonA2 carry out both TG and previously reported (13)] and wider than wild-type cells