Figure 7. Modeling the Impact of Chromo-some-Generated Turgor Pressure on Septal PG and Forespore Size.

(A) Simulation snapshots of differing stages of engulfment at increasing pressure difference (Δp) between the forespore and mother cell. Time between frames is 0.28 hr.

(B) Simulation snapshots for Δp in the range from 8.63–86.3 kPa. All forespores have the same amount of extended septal PG. For these pressures, peptide deformations are in the linear elastic regime, with negligible nonlinear peptide deformation (see Figure S7A).

(C) Simultaneous degradation of SpoIIIE in the mother cell and forespore produces sporangia with varying amounts of DNA in the forespore.

(D) Fluorescence microscopy showing a wild-type strain (wild-type), SpoIIIE^ATP−, and a strain in which SpoIIIE is simultaneously degraded in the mother cell and the forespore (SpoIIIE^Deg.). Scale bar, 1 μm. Arrowheads represent forespores with completely translocated (white), non-translocated (red), partially translocated (yellow), or no (magenta) chromosome.

(E) Fraction of forespore DNA in wild-type sporangia (blue), SpoIIIE^ATP− sporangia (red), and SpoIIIE^Deg. sporangia (green). Each dot represents a different sporangium. Dotted lines, average of each set.

(F) Correlation between the fraction of DNA in the forespore (primary × axis) and forespore volume of the sporangia analyzed in (E). Binned data for all strains are shown with black dots (error bars represent standard deviation) and compared with the simulation (black line) for different Δp (secondary × axis). Vertical dotted lines indicate the average fraction of DNA in SpoIIIE^ATP− (left line) and wild-type (right line) forespores.

(G) Model showing engulfment and reshaping of the forespore. After polar septation, the forespore is hemispherical and is separated from the mother cell by relaxed septal PG (dark gray). Chromosome translocation increases forespore turgor pressure, stretching the septal PG and allowing the accommodation of newly synthesized membrane (red patches on the membrane). Simultaneously, coordinated PG synthesis by forespore biosynthetic proteins (PG, green) and degradation by SpoIIDMP (yellow Pacman) moves the junction between the septum and the lateral cell wall (pink peptide crosslink) around the forespore, extending the septal PG (with new PG represented in green). In the absence of chromosome translocation, forespore turgor pressure remains low, preventing stretching of the extended septal PG and the accommodation of the excess membrane produced during engulfment.

See also Figure S6 and S7.