Ribonuclease T: new exoribonuclease possibly involved in end-turnover of tRNA.

Examination of double mutants lacking one of the exoribonucleases, RNase II, RNase D, RNase BN, or RNase R, and also devoid of tRNA nucleotidyltransferase has suggested that none of these RNases participates in the end-turnover of tRNA. This prompted a search for and identification of a new exoribonuclease, termed RNase T. RNase T could be detected in mutant Escherichia coli strains lacking as many as three of the known exoribonucleases, and it could be separated from each of the four previously described RNases. RNase T is optimally active at pH 8-9 and requires a divalent cation for activity. The enzyme is sensitive to ionic strengths greater than 50 mM and is rapidly inactivated by heating at 45 degrees C. Its preferred substrate is tRNA-C-C-[14C]A, with much less activity shown against tRNA-C-C. RNase T is an exoribonuclease that initiates attack at the 3' hydroxyl terminus of tRNA and releases AMP in a random mode of hydrolysis. The possible involvement of RNase T in end-turnover of tRNA and in RNA metabolism in general are discussed.