Figure 7. Model of the Stalk PG Biosynthetic Complex in A. biprosthecum.

(A) Dimorphic life cycle of A. biprosthecum. The prosthecate mother cell produces an adhesive holdfast (shown in gray) at one pole. Cell division results in a motile, nonreplicating swarmer cell that differentiates into a prosthecate cell. Green circles indicate SpmX localization. Yellow circles represent BacA localization. Numbers in parentheses, (1)-(4), indicate stages of WT stalk synthesis as described in Figure 7B. (B) Model of A. biprosthecum WT stalk synthesis. (1) SpmX (green) appears early in the cell cycle, prior to cell division, where it marks the future site of stalk synthesis. (2) SpmX recruits the putative PG remodeling complex (blue) and the BacA scaffold (yellow) to the site of stalk synthesis. (3) PG synthesis is initiated at the site defined by SpmX localization and in an area constrained by BacA. (4) As PG synthesis progresses and the stalk elongates, synthesis is constrained at the base of the stalk by the BacA scaffold. Timing of these stages in the cell cycle are indicated by the matching numbers in Figure 7A. (C) Model of A. biprosthecum ΔbacA pseudostalk synthesis. As with WT cells, (1) SpmX (green) appears early in the cell cycle, prior to cell division, where it marks the future site of stalk synthesis. (2) SpmX recruits the putative PG remodeling complex (blue), but without BacA. (3) PG synthesis is initiated at the site defined by SpmX localization. (4) As PG synthesis progresses, there is no BacA scaffold to constrain the complex at the base of the synthesis site; PG synthesis occurs promiscuously throughout, with SpmX localized to the distal end of the structure, leading to short, wide pseudostalks. Timing of these stages in the cell cycle are indicated by the matching numbers in Figure 7A.