Figure 1.

The type III secretion system is expressed in almost all Y. enterocolitica, both under secreting and non-secreting conditions. (A) Schematic overview of the different substructures of the T3SS (left). Positions of the labeled proteins analyzed in this study are indicated (right). exp. app, export apparatus. (B) Micrographs of Y. enterocolitica ΔHOPEMTasd expressing EGFP-SctD (top) or EGFP-SctQ (bottom) from their native genetic localization. Three hours after induction of T3SS expression by temperature shift to 37°C under non-secreting conditions, bacteria were subjected to secreting conditions, and imaged. Left, EGFP fluorescence (insets show fluorescence intensity for one enlarged bacterium in ImageJ red-hot coloring scale); center, corresponding phase contrast images; right, overlay. Larger fields of view and images of bacteria under non-secreting conditions are shown in Supplementary Figure 1. (C) Fraction of bacteria with standard expression and distribution of T3SS (multiple membrane foci) for the indicated fusion protein, 3 h after induction of expression of the T3SS under non-secreting conditions (empty bars) or secreting conditions (filled bars) n = 344–388. Numbers on top indicate the number of bacteria that do not display multiple visible T3SS, and the number of analyzed bacteria. Secreting and non-secreting conditions refer to incubation in medium with addition of 5 mM EGTA or CaCl₂, respectively.