Regulation of biosynthesis of aminoacyl-tRNA synthetases and of tRNA in Escherichia coli. III. Biochemical characterization of regulatory mutants affecting leucyl-tRNA synthetase levels

Aminoacyl-tRNA Synthetases in the Bacterial World

Aminoacyl-tRNA synthetases (aaRSs) are modular enzymes globally conserved in the three kingdoms of life. All catalyze the same two-step reaction, i.e., the attachment of a proteinogenic amino acid on their cognate tRNAs, thereby mediating the correct expression of the genetic code. In addition, some aaRSs acquired other functions beyond this key role in translation. Genomics and X-ray crystallography have revealed great structural diversity in aaRSs (e.g., in oligomery and modularity, in ranking into two distinct groups each subdivided in 3 subgroups, by additional domains appended on the catalytic modules). AaRSs show huge structural plasticity related to function and limited idiosyncrasies that are kingdom or even species specific (e.g., the presence in many Bacteria of non discriminating aaRSs compensating for the absence of one or two specific aaRSs, notably AsnRS and/or GlnRS). Diversity, as well, occurs in the mechanisms of aaRS gene regulation that are not conserved in evolution, notably between distant groups such as Gram-positive and Gram-negative Bacteria. The review focuses on bacterial aaRSs (and their paralogs) and covers their structure, function, regulation, and evolution. Structure/function relationships are emphasized, notably the enzymology of tRNA aminoacylation and the editing mechanisms for correction of activation and charging errors. The huge amount of genomic and structural data that accumulated in last two decades is reviewed, showing how the field moved from essentially reductionist biology towards more global and integrated approaches. Likewise, the alternative functions of aaRSs and those of aaRS paralogs (e.g., during cell wall biogenesis and other metabolic processes in or outside protein synthesis) are reviewed. Since aaRS phylogenies present promiscuous bacterial, archaeal, and eukaryal features, similarities and differences in the properties of aaRSs from the three kingdoms of life are pinpointed throughout the review and distinctive characteristics of bacterium-like synthetases from organelles are outlined.

Aminoacyl-tRNA Synthetases in the Bacterial World

Aminoacyl-tRNAsynthetases (aaRSs) are modular enzymesglobally conserved in the three kingdoms of life. All catalyze the same two-step reaction, i.e., the attachment of a proteinogenic amino acid on their cognate tRNAs, thereby mediating the correct expression of the genetic code. In addition, some aaRSs acquired other functions beyond this key role in translation.Genomics and X-ray crystallography have revealed great structural diversity in aaRSs (e.g.,in oligomery and modularity, in ranking into two distinct groups each subdivided in 3 subgroups, by additional domains appended on the catalytic modules). AaRSs show hugestructural plasticity related to function andlimited idiosyncrasies that are kingdom or even speciesspecific (e.g.,the presence in many Bacteria of non discriminating aaRSs compensating for the absence of one or two specific aaRSs, notably AsnRS and/or GlnRS).Diversity, as well, occurs in the mechanisms of aaRS gene regulation that are not conserved in evolution, notably betweendistant groups such as Gram-positive and Gram-negative Bacteria.Thereview focuses on bacterial aaRSs (and their paralogs) and covers their structure, function, regulation,and evolution. Structure/function relationships are emphasized, notably the enzymology of tRNA aminoacylation and the editing mechanisms for correction of activation and charging errors. The huge amount of genomic and structural data that accumulatedin last two decades is reviewed,showing how thefield moved from essentially reductionist biologytowards more global and integrated approaches. Likewise, the alternative functions of aaRSs and those of aaRSparalogs (e.g., during cellwall biogenesis and other metabolic processes in or outside protein synthesis) are reviewed. Since aaRS phylogenies present promiscuous bacterial, archaeal, and eukaryal features, similarities and differences in the properties of aaRSs from the three kingdoms of life are pinpointedthroughout the reviewand distinctive characteristics of bacterium-like synthetases from organelles are outlined.

Increased levels of threonyl-tRNA synthetase in a borrelidin-resistant Chinese hamster ovary cell line

The growth of Chinese hamster ovary cells in medium containing reduced concentrations of threonine is inhibited by borrelidin, a macrolide antibiotic. Borrelidin-resistant clones have been isolated after ethyl methanesulfonate mutagenesis. One clone, 1C-1, has a 3-fold increased level of threonyl-tRNA synthetase [L-threonine:tRNAThr ligase (AMP-forming), EC 6.1.1.3] as determined by both activity measurements and antiserum titrations. The levels of four other aminoacyl-tRNA synthetases and of tRNAThr are the same in strain 1C-1 and in the wild-type parent. The phenotype of increased threonyl-tRNA synthetase activity is recessive to wild type in cell hybrids.

Genetic analysis of mutations causing borrelidin resistance by overproduction of threonyl-transfer ribonucleic acid synthetase

Mutations leading to borrelidin resistance in Escherichia coli by overproduction of threonyl-transfer ribonucleic acid synthetase were anaylzed genetically. The regulatory mutations were closely linked to the treonyl-transfer ribonucleic acid synthetase structural gene (thrS), located clockwise to it. The mutation that causes the threefold-increased enzyme level was more distant from thrS than the mutation responsible for the ninefold overproduction. Both mutations were cis dominant in merodiploid strains, indicating that they affected promoter-operator-like control elements. Overproduction was restricted to threonyl-transfer ribonucleic acid synthetase and was not observed for the products of genes neighboring thrS (e.g., infC, pheS, pheT, and argS), providing evidence that thrS is transcribed singly and that gene amplificationis not a likely basis for increased thrS experession.

Regulation of the biosynthesis of aminoacyl-transfer ribonucleic acid synthetases and of transfer ribonucleic acid in Escherichia coli. VI. Mutants with increased levels of glutaminyl-transfer ribonucleic acid synthetase and of glutamine transfer ribonucleic acid

Spontaneous revertants of a temperature-sensitive Escherichia coli strain bearing a thermolabile glutaminyl-transfer ribonucleic acid (tRNA) synthetase have been selected for growth at 45 degrees C. Among 10 revertants still containing the thermolabile enzyme, 2 interesting strains were found. One strain has a fivefold elevated level of the thermolabile glutaminyl-tRNA synthetase; the genetic locus, glnR, responsible for this effect maps at min 24, far from glnS, the structural gene of the enzyme. In the other strain the levels of tRNA Gln and several other tRNAs are twice as high as in the parental strain; the locus responsible, glnU, maps at min 59.5 on the E. coli map.