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Mutations in Residues Involved in Zinc Binding in the Catalytic Site of Escherichia coli Threonyl-tRNA Synthetase Confer a Dominant Lethal Phenotype

CNRS UPR9073, Université de Paris VII, Institut de Biologie Physico-Chimique, 13 rue Pierre et Marie Curie, 75005 Paris, France,¹ Architecture et Réactivité de l'ARN, Université Louis Pasteur de Strasbourg, CNRS, Institut de Biologie Moléculaire et Cellulaire, 15 rue René Descartes, 67084 Strasbourg, France²

Received 25 March 2007/ Accepted 16 July 2007

Escherichia coli threonyl-tRNA synthetase is a homodimeric protein that acts as both an enzyme and a regulator of gene expression: the protein aminoacylates tRNA^Thr isoacceptors and binds to its own mRNA, inhibiting its translation. The enzyme contains a zinc atom in its active site, which is essential for the recognition of threonine. Mutations in any of the three amino acids forming the zinc-binding site inactivate the enzyme and have a dominant negative effect on growth if the corresponding genes are placed on a multicopy plasmid. We show here that this particular property is not due to the formation of inactive heterodimers, the titration of tRNA^Thr by an inactive enzyme, or its misaminoacylation but is, rather, due to the regulatory function of threonyl-tRNA synthetase. Overproduction of the inactive enzyme represses the expression of the wild-type chromosomal copy of the gene to an extent incompatible with bacterial growth.

Published ahead of print on 20 July 2007.