Studies of becterial chemotaxis to oxygen (aerotaxis) over a broad range of oxygen concentratons showed that at high concentrations, oxygen was a repeflent of SabnoneUa typhiinurium, Escheichia coio, and some bacilli, whereas it is known that at lower concentrations (0.25 mM dissolved oxygen), oxygen Is an atratant.In a temporal assay of aerotaxis, S. typhimurium in nedium equilbrated with air (0.25 mM dissoved oxygen) and then exposed to pure oxygen (1. Aerotaxis is a migrational response of microorganisms to a gradient of oxygen. tarly studies of aerotaxis were made by generating an oxygen gradient in situ and observing the niigration of a population of bacteria. Beijerinck placed 1 drop of a bacterial culture beneath a cover slip, and the bacteria consumed the oxygen in the medium and then migrated in response to oxygen diffusing in front the edges of the cover slip (3). Obligate aerobes formed a band at the edge of the cover slip, microaerophilic spirilla formned a band that was some distance in from the edge, and anaerobes collected in the center of the cover slip. Engelmann placed bacteria in a vessel with illuminated algae (6). The bacteria accumulated near the algal chloroplasts, where oxygen was formed. Spirilla were characterized by a cell-free zone between them and the oxygen source. If the illumination was sufficiently intense, the cells were repelled entirely.Expanding on the early studies of aerotaxis, Baracchini and Sherris screened 24 mtotile species of bacteria for aerotaxis by using the algal assay and a capillary assay (2). In the capillary assay a pellet of bacteria was placed at one end of a liquid-filled tube, and the end was sealed. In response to the oxygen gradient generated by respiration, a band of bacteria migrated along the length of the capillary.

Mutants ofEscherichia coli and Salmonella typhimurium that were deficient in protein methylesterase activity encoded by cheB had an inverted response to oxygen; they were repelled by concentrations of oxygen that attract wild-type bacteria. Normal responses to oxygen and phosphotransferase substrates were observed in mutants that were deficient in protein methyltransferase (CheR) and the methyl-accepting transducing proteins (Tsr, Tar, Trg). However, the methylation-independent response to oxygen was modified by the loss of esterase activity. The inversion was apparently effected by the amidated Tsr protein present in cheB tsr+ mutants because aerotaxis was normal in cheB tsr strains. Chemotaxis to phosphotransferase sugars was normal in cheB mutants provided the extreme clockwise bias of the flagellar motors was modified to increase the probability of counterclockwise rotation.Chemotaxis by Salmonella typhimurium and Escherichia coli toward most chemoeffectors involves methyl-accepting transducing proteins that span the cytoplasmic membrane (11,12,21,30,37). Attractant, or an attractant-receptor complex if the primary receptor is a soluble periplasmic protein, binds to a transducing protein at the outer surface, probably inducing a conformational change that is propagated to the interior of the cell and signals a decrease in the probability of clockwise rotation of the flagellar motors. The transducing protein is subsequently methylated by the cheR product, protein methyltransferase, and the net effect is to cancel the excitation signal so that the bacteria adapt to the new concentration of attractant (5, 31). If the attractant is withdrawn or a repellent is added to the adapted cells, the transducing protein sends a signal to the flagellar motor that greatly increases the probability of clockwise rotation. The transducing protein is then demethylated by the cheB product, protein methylesterase, until the signal for clockwise rotation is cancelled and the bacteria adapt (5, 33). The CheB protein is also involved in processing newly synthesized transducing proteins by glutaminyl residue deamidation which yields sites for methylesterification (8,25). Three known methyl-accepting transducing proteins are the products of the tsr, tar, and trg genes (6,10,27,29). The tap gene has extensive homology to tsr and tar, but no transducing role for the Tap protein has been demonstrated (12, 28).In addition to most chemoeffectors that act on methylaccepting transducing proteins, there are some attractants that are recognized by sensory transduction systems that are independent of methylation (19). Sugar substrates for the phosphotransferase transport system are attractants for one methylation-independent chemotactic system in which the enzyme II receptor for transport is also the receptor for chemotaxis (1). Adaptation to phosphotransferase sugars occurs normally in cells with impaired protein methylation * Corresponding author. t Present address: Snow-Brand Milk Products Co., Ltd., Research Institute of Life Science, 519, ...

Spontaneously occurring autoantibody to threonyl-tRNA synthetase found in the serum of patients with polymyositis and experimentally induced antibody against highly purified rabbit reticulocyte threonyl-tRNA synthetase were used to analyze the epitopes of threonyl-tRNA synthetase. The PL-7 autoantibody reacted with the native but not the denatured form of threonyl-tRNA synthetase, whereas the experimentally induced antibody recognized both the native and denatured forms of the enzyme. In addition, the PL-7 autoantibody specifically inhibited threonyl-tRNA synthetase activity whereas the experimentally induced antibody had no effect on aminoacylation. Thus, the epitopes recognized by the PL-7 autoantibody are formed by the tertiary structure of the enzyme and are associated with the catalytic site of the synthetase whereas the experimentally induced antibody recognizes epitopes formed by primary sequences not related to the catalytic function of the synthetase.

The aminoacyl-tRNA synthetases play a dual role in cell metabolism by synthesizing aminoacyl-tRNAs and an odd dinucleotide diadenosine-5', 5''-P1, P4-tetraphosphate which appears to be involved in DNA replication and the control of cell proliferation. This review is a synthesis of recent results on the structure, genetics, cell biology, physiology, role in neoplasia, and role in autoimmune myositis of the higher eukaryotic aminoacyl-tRNA synthetases.