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J Biol Chem, Vol. 274, Issue 45, 32445-32452, November 5, 1999

Characterization of the Gene Encoding Serine Acetyltransferase, a Regulated Enzyme of Cysteine Biosynthesis from the Protist Parasites Entamoeba histolytica and Entamoeba dispar
REGULATION AND POSSIBLE FUNCTION OF THE CYSTEINE BIOSYNTHETIC PATHWAY IN ENTAMOEBA

Tomoyoshi Nozaki^§, Takashi Asai^§, Lidya B. Sanchez, Seiki Kobayashi^§, Miki Nakazawa^§, and Tsutomu Takeuchi^§

From the  Department of Parasitology, National Institute of Infectious Diseases, 1-23-1 Toyama, Shinjuku-ku, Tokyo 162-8640, Japan, the ^§ Department of Tropical Medicine and Parasitology, Keio University School of Medicine, 35 Shinanomachi, Shinjuku-ku, Tokyo 160-8582, Japan, and  The Rockefeller University, New York, New York 10021

The enteric protist parasites Entamoeba histolytica and Entamoeba dispar possess a cysteine biosynthetic pathway, unlike their mammalian host, and are capable of de novo production of L-cysteine. We cloned and characterized cDNAs that encode the regulated enzyme serine acetyltransferase (SAT) in this pathway from these amoebae by genetic complementation of a cysteine-auxotrophic Escherichia coli strain with the amoebic cDNA libraries. The deduced amino acid sequences of the amoebic SATs exhibited, within the most conserved region, 36-52% identities with the bacterial and plant SATs. The amoebic SATs contain a unique insertion of eight amino acids, also found in the corresponding region of a plasmid-encoded SAT from Synechococcus sp., which showed the highest overall identities to the amoebic SATs. Phylogenetic reconstruction also revealed a close kinship of the amoebic SATs with cyanobacterial SATs. Biochemical characterization of the recombinant E. histolytica SAT revealed several enzymatic features that distinguished the amoebic enzyme from the bacterial and plant enzymes: 1) inhibition by L-cysteine in a competitive manner with L-serine; 2) inhibition by L-cystine; and 3) no association with cysteine synthase. Genetically engineered amoeba strains that overproduced cysteine synthase and SAT were created. The cysteine synthase-overproducing amoebae had a higher level of cysteine synthase activity and total thiol content and revealed increased resistance to hydrogen peroxide. These results indicate that the cysteine biosynthetic pathway plays an important role in antioxidative defense of these enteric parasites.

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Copyright © 1999 by The American Society for Biochemistry and Molecular Biology, Inc.

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