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Volume 272, Number 5, Issue of January 31, 1997 pp. 2744-2752
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.

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Siroheme Biosynthesis in Higher Plants
ANALYSIS OF AN S-ADENOSYL-L-METHIONINE-DEPENDENT UROPORPHYRINOGEN III METHYLTRANSFERASE FROM ARABIDOPSIS THALIANA

(Received for publication, May 22, 1996, and in revised form, August 29, 1996)

Thomas Leustek , Michael Smith , Michael Murillo , Davinder Pal Singh , Alison G. Smith , Sarah C. Woodcock ^** , Sarah J. Awan ^** and Martin J. Warren ^**

From the  Center for Agricultural Molecular Biology and Plant Science Department, Rutgers University New Brunswick, New Jersey 08903-0231,  Department of Plant Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EA, United Kingdom, and ^** Department of Molecular Genetics, Institute of Ophthalmology, University College London, Bath Street, London EC1V 9EL, United Kingdom

Siroheme, the prosthetic group for both nitrite and sulfite reductases, is a methylated, iron-containing modified tetrapyrrole. Here we report the first molecular characterization of the branch point enzyme in higher plants, which directs intermediates toward siroheme synthesis. A cDNA was cloned from Arabidopsis thaliana (UPM1) that functionally complements an Escherichia coli cysG mutant, a strain that is unable to catalyze the conversion of uroporphyrinogen III (Uro'gen-III) to siroheme. UPM1 is 1484 base pairs and encodes a 369-amino acid, 39.9-kDa protein. The UPM1 product contains two regions that are identical to consensus sequences found in bacterial Uro'gen-III and precorrin methyltransferases. Recombinant UPM1 protein was found to catalyze S-adenosyl-L-methionine-dependent transmethylation by UPM1 in a multistep process involving the formation of a covalently linked complex with S-adenosyl-L-methionine. The UPM1 product has a sequence at the amino terminus that resembles a transit peptide for localization to mitochondria or plastids. The protein produced by in vitro expression is able to enter isolated intact chloroplasts but not mitochondria. Genomic blot analysis showed that UPM1 is encoded in the A. thaliana genome. The genomic DNA corresponding to UPM1 was cloned and sequenced and found to contain at least five introns.

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