ISOLATION AND CHARACTERIZATION OF A CDNA THAT ENCODES MAIZE UROPORPHYRINOGEN-III METHYLTRANSFERASE, AN ENZYME INVOLVED IN THE SYNTHESIS OF SIROHEME, WHICH IS A PROSTHETIC GROUP OF NITRITE REDUCTASE

A full-length cDNA clone, pZmSUMT1, encoding an S-adenosyl-L-methionin e-dependent uroporphyrinogen III C-methyltransferase (SUMT; EC 2.1.1.1 07) of maize was isolated from a root cDNA library. pZmSUMT1 had an in sert of 1.7 kb and the amino acid sequence deduced from the open readi ng frame of the cDNA was similar to that of SUMT from various bacteria and also to the SUMT catalytic region of siroheme synthase (cysG) fro m Escherichia coil. Overproduction of ZmSUMT1 in a cysG mutant of E. c oil eliminated the requirement of the strain for cysteine. Transcripts for ZmSUMT1 accumulated rapidly in both roots and leaves in response to the addition of nitrate to the culture medium. The effects of bioch emical inhibitors on the nitrate-dependent induction of the gene for Z mSUMT1 coincided with the effects on the genes for other nitrate-assim ilatory enzymes, nitrate reductase and nitrite reductase. An import ex periment in vitro suggested that the gene product might be located in plastids. The results together indicate that ZmSUMT1 might be involved in the synthesis of siroheme, a prosthetic group of nitrite reductase , and that the expression of its gene is coregulated with that of othe r nitrate-assimilatory genes.