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The Bacterial Carbon-Fixing Organelle Is Formed by Shell Envelopment of Preassembled Cargo

Figure 1

Carboxysomes are born one at a time at the site of preexisting carboxysomes.

(A) In pulse-chase labeling of RbcL-SNAP in live S. elongatus cells, actively assembling carboxysomes with solvent accessible RbcL-SNAP are labeled with BG dye. Red: phase contrast. Green: RbcL. Scale bar: 1µm. (B) The distribution of the number of SNAP labeled carboxysomes, indicating active assembly, in cells directly after labeling (n=442). (C) The biogenesis of carboxysomes can be monitored from long timelapses. Red: phase contrast. Green: RbcL-GFP. Scale bar: 1µm. (D) Montage showing the formation of new carboxysome at the site of a preexisting carboxysome. White arrow indicates the birth event. Panel height: 25 pixels. Time interval: 3 minutes. (E–F) RuBisCO foci elongate into bar carboxysomes that subsequently split into two carboxysomes. Scale bar: 1µm. Time interval: 75 minutes. (G–I) Kymographs of RbcL-GFP in growing and dividing cells. Carboxysome birth events are indicated by white arrows. Scale bar: 1µm. Time interval: 3 minutes. (J) Spatial distribution of 234 birth events along the long axis of the cell. Quarter cell positions are favored. (K) Relative intensity of 141 pairs of new (daughter) carboxysomes and the preexisting carboxysomes to which they initially colocalize (mothers) reveals that birth events are highly asymmetric, with mean daughter intensity being 1/4 that of the mother. Because pairs are sorted into dim (daughter) and bright (mother) pairs, no data points can fall into the shaded area. Dotted line indicates a 1:4 ratio.

Figure 1