Fig. 4.

The toga as a model for OM biogenesis. (A) Major evolutionary steps outlining a proposed mechanism for OM biogenesis: 1) the primordial cell was likely monoderm, 2) PG and/or a protein surface layer evolved to support the bilayer of the primordial cell, 3) the abundance of surface proteins was able to support small membrane patches (analogous to the toga), 4) lipid accumulation extended to form a second membrane. Since the β-barrel fold (blue) is ancient, it likely evolved concurrently with the OM. (B) A phylogenetic tree of Terrabacteria depicting the major early-branching phyla Witwinowski et al. 7. Diversity in the cell envelopes could be explained with losses and gains of the OM as shown as red and green arrows for Firmicutes and Actinobacteria, respectively. Membrane architecture annotations are based on cryo-ET studies (except CPR, uncultured). Presence of OM signature features such as Omp85/BamA superfamily, LPS synthetic pathway, SlpA/OmpM tethering system, is indicated with black circles. (C) Cell envelope model for T. maritima based on current study showing major structural components of the toga. The SlpA/OmpM-like tethering system is likely composed of multiple Ompα and β-barrel proteins.