A series of nested N-terminal deletions were made on the full-length (wt) and C-terminal deleted (Cdel) 1-aminocyclopropane-1-carboxylate synthase cDNAs. These wt and mutant ACC synthases were over-expressed in a heterologous E. coli expression system. It was found that removal of an amino acid region (residues 2–12) from the non-conserved N-termini of wt and Cdel ACC synthases led to a slight increase in both in vivo ACC production and in vitro ACC synthase activity. Further deletion of 11 amino acids through Glu-23 from the N-termini of both wt and Cdel ACC synthases resulted in a substantial reduction in both in vivo ACC production and in vitro enzyme activity. Deletion of an amino acid region, residues 3 through 27, from the N-terminus of ACC synthase abolished enzyme activity completely. Kinetic analysis of a highly purified double-deletion mutant (NCdel-1) of ACC synthase demonstrated that the K m of this mutant is 42 μM, which is much smaller than that of the corresponding Cdel (280 μM) and closer to that of wt (22 μM) reported previously, suggesting a clear effect of the non-conserved N-terminal region on its ACC synthase function.
