THE TRANSFER-RNA PROCESSING ENZYME RNASE-T IS ESSENTIAL FOR MATURATION OF 5S RNA

The maturation of 5S RNA in Escherichia coli is poorly understood. Alt hough it is known that large precursors of 5S RNA accumulate in mutant cells lacking the endoribonuclease RNase E, almost nothing is known a bout how the mature 5' and 3' termini of these molecules are generated . We have examined 5S RNA maturation in wild-type and single- or multi ple-exoribonuclease-deficient cells by Northern blot and primer-extens ion analysis. Our results indicate that no mature 5S RNA is made in RN ase T-deficient strains. Rather, 5S RNA precursors containing predomin antly 2 extra nucleotides at the 3' end accumulate. Apparently, these 5S RNAs are functional inasmuch as mutant cells are viable, growing on ly slightly slower than wild type. Purified RNase T can remove the ext ra 3' residues, showing that it is directly involved in the trimming r eaction. In contrast, mutations affecting other 3' exoribonucleases ha ve no effect on 5S RNA maturation. Approximately 90% of the 5S RNAs in both wild-type and RNase T- cells contain mature 5' termini, indicati ng that 5' processing is independent of RNase T action. These data ide ntify the enzyme responsible for generating the mature 3' terminus of 5S RNA molecules and also demonstrate that a completely processed 5S R NA molecule is not essential for cell survival.