"Activation complexes" or complexes of aminoacyl-tRNA synthetases (amino acid:tRNA ligases, E.C.6.1.1.) sedimented from rat liver homogenates as postmicrosomal particles will be further investigated. The objective is to acquire information concerning the properties and structure of these particles and their possible origin in a supramolecular structure in the living cell, responsible for the rapid and specific production of protein. The heterogeneity of rat liver postmicrosomal particles will be examined with respect to particle size and distribution of aminoacyl-tRNA synthetase content. The associated RNA will be examined for heterogeneity. Particles will be separated by sedimentation on sucrose density gradients, and possibly by gel filtration on Sepharose. A large particle (less than 50S) will be purified by conventional methods. The interaction of complexes of synthetases with polysomes and microsomes will be studied. Evidence will be sought for specific associations between the complex of synthetase and polysomes and microsomes. This will be done by electron microscopy, and by a study of the kinetics and certain cofactor requirements of the system for in vitro protein synthesis. The occurance of such interactions might provide some clue as to the common origin of the different particles in a supramolecular complex. The reactions catalysed by the complex of synthetases and soluble synthetases will be studied, to ascertain if the two forms of synthetase differ in their properties as catalysts.