Distribution of Stx1 and Stx2 and effect of overexpressed histidine-tagged StxB in EHEC. EHEC K24 (Stx1), EHEC EDL933 (Stx1 and Stx2), EHEC 86-24 (Stx2), and transformed EHEC 86-24 were fractionated into supernatant and cell-associated fractions. Each 2 μg of protein of the cell-associated fraction (P) and an equal volume of the supernatant fraction (S) were analyzed by SDS-PAGE and immunoblotting using a mixture of anti-Stx1 antiserum, anti-Stx2 antiserum, and anti-His tag antibody. Values represent the ratios of StxA2 protein levels to the cell-associated fraction in the supernatant fraction (upper) and the ratios of StxB-H protein levels to the supernatant fraction in the cell-associated fraction (lower).