Fig. 1. Hypoxia blocks culmination in the normal Dictyostelium strain Ax3. (A) Cells were developed for 25 hours on filters in an atmosphere of the indicated percentage of O₂ with N₂. Similar results were obtained in the presence of 1% CO₂ (not shown). (B) Timecourse of effect of O₂ shift on culmination visualized qualitatively at 20 hours. (C) Effect of hypoxia on cell proliferation. Ax3, P4H1^oe (HW285) or P4H1^- (HW288) cells (see later) were diluted to 10⁶ cells/ml (indicated by dashed line) in growth medium (HL-5) and shaken in the indicated percent of O₂ in N₂. Cells were counted 24 hours later. (D) Effect of overexpressing PKAcat on the hypoxic blockade, quantitated by counting detergent-resistant spores after 36 hours. Spore numbers correlated with morphological culmination (not shown). Results are typical of three independent trials.