The modular structure of Escherichia coli threonyl-tRNA synthetase as both an enzyme and a regulator of gene expression.
In addition to its role in tRNA aminoacylation, Escherichia coli threonyl-tRNA synthetase is a regulatory protein which binds a site, called the operator, located in the leader of its own mRNA and inhibits translational initiation by competing with ribosome binding. This work shows that the two essential steps of regulation, operator recognition and inhibition of ribosome binding, are performed by different domains of the protein. The catalytic and the C-terminal domain of the protein are involved in binding the two anticodon arm-like structures in the operator whereas the N-terminal domain of the enzyme is responsible for the competition with the ribosome. This is the first demonstration of a modular structure for a translational repressor and is reminiscent of that of transcriptional regulators. The mimicry between the operator and tRNA, suspected on the basis of previous experiments, is further supported by the fact that identical regions of the synthetase recognize both the operator and the tRNA anticodon arm. Based on these results, and recent structural data, we have constructed a computer-derived molecular model for the operator-threonyl-tRNA synthetase complex, which sheds light on several essential aspects of the regulatory mechanism.
Caillet
Joël
J
UPR9073 du CNRS, Institut de Biologie Physico-Chimique, 13 rue Pierre et Marie Curie, 75005 Paris, France.
Nogueira
Teresa
T
Masquida
Benoît
B
Winter
Flore
F
Graffe
Monique
M
Dock-Brégeon
Anne-Catherine
AC
Torres-Larios
Alfredo
A
Sankaranarayanan
Rajan
R
Westhof
Eric
E
Ehresmann
Bernard
B
Ehresmann
Chantal
C
Romby
Pascale
P
Springer
Mathias
M
D001665:Binding Sites; D001667:Binding, Competitive; D004926:Escherichia coli / Q000201:enzymology* / Q000235:genetics*; D019143:Evolution, Molecular; D015964:Gene Expression Regulation, Bacterial; D005798:Genes, Bacterial; D046911:Macromolecular Substances; D008958:Models, Molecular; D018716:Molecular Mimicry; D015394:Molecular Structure; D009154:Mutation; D009875:Operator Regions, Genetic; D017434:Protein Structure, Tertiary; D021122:Protein Subunits; D012329:RNA, Bacterial / Q000737:chemistry / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D012346:RNA, Transfer, Amino Acyl / Q000737:chemistry / Q000378:metabolism; D012270:Ribosomes / Q000378:metabolism; D013914:Threonine-tRNA Ligase / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*
D046911:Macromolecular Substances; D021122:Protein Subunits; D012329:RNA, Bacterial; D012333:RNA, Messenger; D012346:RNA, Transfer, Amino Acyl; D013914:Threonine-tRNA Ligase