Zinc ion effects on individual Ssp DnaE intein splicing steps:: Regulating pathway progression

Use of the naturally split, self-splicing Synechocystis sp. PCC6803 DnaE intein permits separate purification of the N- and C-terminal intein domains. Otherwise spontaneous intein-mediated reactions can therefore be controlled in vitro, allowing detailed study of intein kinetics. Incubation of the Ssp DnaE intein with ZnCl2 inhibited trans splicing, hydrolysis-mediated N-terminal trans cleavage, and C-terminal trans cleavage reactions. Maximum inhibition of the splicing reaction was achieved at equal molar concentrations of ZnCl2 and intein domains, suggesting a 1:1 metal ion:intein binding stoichiometry. Mutation of the (+)1 cysteine residue to valine (C(+)1V) alleviated the inhibitory effects of ZnCl2. Valine substitution in the absence of ZnCl2 blocked trans splicing and decreased C-terminal cleavage kinetics in a manner similar to that of the native (+)1 cysteine in the presence of ZnCl2. These data are consistent with Zn2+-mediated inhibition of the Ssp DnaE intein via chelation of the (+)1 cysteine residue. N-Terminal trans cleavage can occur via both spontaneous hydrolysis and nucleophilic (e.g., DTT) attack. Comparative examination of N-terminal cleavage rates using amino acid substitution (C(+)1V) and Zn2+-mediated inhibition permitted the maximum contribution of hydrolysis to overall N-terminal cleavage kinetics to be determined. Stable intermediates consisting of the associated intein domains were detected by PAGE and provided evidence of a rapid C-terminal cleavage step. Acute control of the C-terminal reaction was achieved by the rapid reversal of Zn2+-mediated inhibition by EDTA. By inhibiting both the splicing pathway and spontaneous hydrolysis with Zn2+, reactants can be diverted from the trans splicing to the trans cleavage pathway where DTT and EDTA can regulate N- and C-terminal cleavage, respectively.