S-Adenosyl-L-Methionine: uroporphyrinogen III methyltransferase (SUMT), a key regulatory enzyme, converts uroporphyrinogen III to precorrin-2 in the porphinoids biosyntliesis. In this study, the mature SUMT was signified that the maize SUMT precursor encoded by the open reading frame of maize SUMT cDNA was deleted the first 91 amino acids constituting the postulated signal peptide. Several mature SUMT fusion and deletion mutants were conducted. It actively expressed in Escherichia coli that the mature SUMT, or the truncated one deleting the C-terminal extra 52 amino acids based on SUMT sequence comparisons. On the contrary, it expressed as an inclusion body in E coli that the mature SUMT fusion mutant, the SUMT precursor.. or the mature SUMT deleting the N-terminal 36 amino acids including glycine-rich region involved directly in SAM binding. The purified His(6)-tagged mature SUMT was homodimer with a molecular weight of 34 kDa. as shown by SDS-PAGE, 52 kDa using ael-filtration chromatography, and 79 kDa by dynamic light scattering assay. Red fluorescent compounds were associated with the recombinant mature SUMT which were identified as sirohydrochlorin and trimethylpyrrocorphin by spectroscopic analysis. This association slightly altered the protein secondary structure confirmed by circular dichroism assay. (c) 2006 Published by Elsevier Inc.