Studies continued in the area of tRNA transport, biosynthesis, and processing the molecular biology of Alu sequences, and the mechanism of mRNA transport. We have in previous studies described the discovery of a mechanism by which tRNA molecules are transported from the nucleus of eukaryotic cells. The particular features of the tRNA molecule recognized by this transport system was defined by an analysis of 30 single point mutants generated in vitro in the human tRNA met i gene. The studies demonstrated that all mutations generating transport-defective species also generated pre-tRNA species which were inefficiently processed, suggesting that the processing nucleases might be playing a role in transport. The processing nucleases were purified to homogeneity. The 5' processing enzyme was found to be a complex enzyme composed of at least 14 different polypeptides and comprising a cylinder shaped particle. The particle appears to be identical to an ubiquitous subcellular particle previously described in the literature over the past 15 years. Studies on the biology of the Alu sequence have identified the first evidence that these ubiquitous genes generate processed cellular RNAs. The existence of a novel polypeptide which interacts specifically with the Alu RNA has been identified through the use of autoantisera from an individual with lupus. With this antibody, a corresponding polypeptide has been identified in human cells suggesting that analogues of the mouse Bl Alu sequence exist in man. Studies on the mechanism of mRNA transport continue. The principal advance over the past year has been the establishment of a precise system utilizing the X. laevis oocyte to follow the kinetics of movement of mRNA from nucleus to cytoplasm. With this system the transport model we have previously proposed, utilizing a ribosome fixed at the nuclear envelope as the motor, is being tested.
