Identification of a ribonuclease P-like activity from human KB cells

An endoribonuclease which cleaves tRNA precursor molecules has been partially purified from human KB tissue culture cells. This activity is found in cytoplasmic fractions but is not detectable in the nucleoplasm. tRNA precursor molecules from both E. coli and KB cells are cleaved by this novel activity to produce 5′ phosphate-terminated oligonucleotides. E. coli RNAase P and the KB cell nuclease both make a single endonucleolytic scission in E. coli tRNA^Tyr precursor, thereby separating the 41 extra nucleotides on the 5′ end of the precursor molecule from the 5′ terminal sequence of the mature tRNA^Tyr molecule. The cleavage products generated from other E. coli tRNA precursors by the KB cell activity are identical in size to those produced by RNAase P. The KB cell endoribonuclease requires Mg^2- and a monovalent cation (Na⁺, K⁺, or NH₄⁺) for function. The enzymatic activity has a broad pH optimum, centered near pH 8.0, and the activity is inhibited by tRNA. Several KB cell RNAs with long half-lives in vivo, including 5S and bulk 4S RNA, are not cleaved by this nuclease. The KB cell endoribonuclease resembles E. coli RNAase P in its substrate specificity, pH optimum, ion requirements, and sensitivity to tRNA. These properties and the cytoplasmic localization of the novel endoribonuclease indicate its involvement in the biosynthesis of KB cell tRNA.