C, Growth curves for E. coli 113/3 in defined media and in the presence or absence of B12 ( and o, r­­espectively). When challenged with 40 nM isolated R-domain ( ), cell growth was largely impaired whereas growth in the presence of 40 nM ColE9 W39A (▫) a­­pproached that of the no colicin control, consistent with OmpF threading causing the disengagement of ColE9 from BtuB. Error bars represent the standard deviation across three biological replicates. 