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J. Bacteriol. doi:10.1128/JB.00439-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

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 P. et M. Curie, 75005 Paris, France; Architecture et Réactivité de l'ARN, Université Louis Pasteur de Strasbourg, CNRS, IBMC, 15 rue R. Descartes, 67084 Strasbourg, France

^* To whom correspondence should be addressed. Email: Mathias.Springer{at}ibpc.fr.

	Abstract

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.