SUBUNIT STRUCTURE AND FUNCTION OF MICROCOCCUS CRYOPHILUS GLUTAMYL TRANSFER RNA SYNTHETASE

Two strains, 68 and TMR 9, of Micrococcus cryophilus differ in their physiological response to environmental temperature: TMR 9 is a mutant strain which will grow at temperatures inimical to the growth of the wild-type strain, 68. These growth responses have been correlated with the presence, respectively, of thermostable and thermolabile species of glutamyl-tRNA synthetase and prolyl-tRNA synthetase in the two strains. Temperature-induced structural changes in the two species of glutamyl-tRNA synthetase have been studied in an attempt to explain the functional differences between them. An isolation procedure for the purification of the stable, TMR 9 enzyme, yielding almost 200-fold purification, and the preparation of antiserum active against this protein, are described. Molecular weight determinations of glutamyl-tRNA synthetase in both 68 and TMR 9 cell extracts have been carried out by gel filtration and sucrose density gradient centrifugation. In both extracts, cross-reacting material and associated enzyme activity were to be found primarily at a position corresponding to a molecular weight of 190 000 with a minor peak at about half that weight. Heating wild-type extracts at 30° prior to chromatography led to the appearance of cross-reacting material, with no associated enzyme activity, with an estimated molecular weight of 53 000, this accumulation being correlated with a loss of activity and cross-reacting material from the 190 000 and 100 000 molecular weight positions. No re-association of these small subunits was observed. The mutant enzyme could also be broken down to inactive subunits of similar weight, but only under extreme conditions. © 1969.