Construction of the NB-Ctb(His) and NB-L2-Ctb(His) expression vectors. Panel A. The expression plasmid pVCU720 was digested with NdeI liberating a DNA fragment encoding the PelB leader sequence fused to the NB fragment and the Ctb A2 fragment. This fragment was inserted into the Ctb(his) expression vector, pVCU721, which was similarly digested with NdeI, creating the NB-Ctb(his) expression plasmid, pVCU722. Panel B. The expression plasmid pVCU724 was digested with NdeI liberating a DNA fragment encoding the following, in order: PelB leader sequence fused to the NB region, followed by the L2 domain, and the A2 region. The NdeI fragment was inserted into the Ctb(his) expression vector pVCU721, which was similarly digested with NdeI, creating the NB-L2-Ctb(his) expression plasmid pVCU725. Plasmids and genes are not drawn to scale.