IMMUNOELECTRON MICROSCOPIC LOCATION OF TRYPTOPHANYL-TRANSFER-RNA SYNTHETASE IN MAMMALIAN, PROKARYOTIC AND ARCHAEBACTERIAL CELLS

Monoclonal antibody Am1 against conservative epitope of trypto- phanyl -tRNA synthetase (WRS) was labeled with colloidal gold particles and u sed to localize the enzyme on ultrathin sections of eubacteria (Escher ichia coli), archaebacteria (Methanococcus halophilus), rat pancreas t issue and rat fibroblasts (cell line RAT1). In all cell types immunoel ectron microscopy revealed predominant cytoplasmic location of gold pa rticles, as this could be expected from known biochemical data. In par ticular, in mammalian cells intensive labeling was observed in cytopla smic regions rich in polysomes and free ribosomes. At the same time, t he label was virtually absent in cytoplasmic regions where microfilame nt bundles were present. Significant concentrations of gold particles were found in mitochondria and nuclei. In the latter case, gold partic les were located over diffuse chromatin regions and were virtually abs ent over compact chromatin. The density of diffuse chromatin in labeli ng may amount to about 50% of that found in the cytoplasm. Distributio n of labeled antibodies over E. coli cells looks rather similar to tha t found for M. halophilus: gold particles are preferably concentrated over the cytoplasm and ''boundary zone'', i.e., a 30 nm wide cytoplasm ic zone adjacent to the nucleoid border, while the label over nucleoid is virtually absent.Two main conclusions are drawn: (i) although in t he animal cell homogenates WRS is recovered mainly as a soluble cytoso lic enzyme, in intact cells it is associated with defined cellular org anelles and compartments; this may be an evolutionarily acquired featu re probably typical for multicellular organisms; (ii) the considerable density of labeling in diffuse (not compact) chromatin regions may be indicative of WRS involvement in the active chromatin functions (tran scription, processing, transfer of gene products, etc.).