Precise excision of intervening sequences from precursor tRNAs by a membrane-associated yeast endonuclease

Splicing of transfer RNA precursors containing intervening sequences proceeds in two distinct stages: endonucleolytic cleavage, followed by ligation. We have physically separated endonuclease and ligase activities from extracts of yeast cells, and we report properties of the partially purified endonuclease preparation. The endonuclease behaves as an integral membrane protein: it is purified from a membrane fraction from which it can be solubilized with nonionic detergents, and the activity of the endonuclease in the membrane fraction is stimulated by nonionic detergents. The endonuclease cleaves precursor tRNAs at two sites to excise the intervening sequence precisely. Both the extent and the accuracy of cleavage are enhanced by the presence of spermidine; the degree of stimulation varies with the pre-tRNA substrate. The cleavage products possess 5'-hydroxyl and 2',3'-cyclic phosphodiester termini. The cyclic phosphodiester termini can be opened to 2'-phosphates by a cyclic phosphodiesterase activity in the preparation.