FIGURE 6.

AsbA and AsbB display varying selectivity for nucleophilic substrates. A MESG-coupled pyrophosphate detection assay compared relative rate of incorporation of linear nucleophiles by AsbA and AsbB. Data are normalized to the preferred substrate spermidine. A, AsbB is more promiscuous by comparison; however, the general trend in substrate tolerance is conserved between the two NIS synthetases. B, residues in the AsbB substrate binding cleft hypothesized to interact with spermidine, Lys-311 and Glu-459, were subjected to site directed mutagenesis. AsbB mutants were compared for relative nucleophile acceptance.