Examining tRNA 3′-ends in Escherichia coli: teamwork between CCA-adding enzyme, RNase T, and RNase R

tRNA maturation and quality control are crucial for proper functioning of these transcripts in translation. In several organisms, defective tRNAs were shown to be tagged by poly(A) or CCACCA tails and subsequently degraded by 3'-exonucleases. In a deep-sequencing analysis of tRNA 3'-ends, we detected the CCACCA tag also in Escherichia coli. However, this tag closely resembles several 3'-trailers of tRNA precursors targeted for maturation and not for degradation. Here, we investigate the ability of two important exonucleases, RNase R and RNase T, to distinguish tRNA precursors with a native 3'-trailer from tRNAs with a CCACCA tag. Our results show that the degrading enzyme RNase R breaks down both tRNAs primed for degradation as well as precursor transcripts, indicating that it is a rather nonspecific RNase. RNase T, a main processing exonuclease involved in trimming of 3'-trailers, is very inefficient in converting the CCACCA-tagged tRNA into a mature transcript. Hence, while both RNases compete for trailer-containing tRNA precursors, the inability of RNase T to process CCACCA tails ensures that defective tRNAs cannot reenter the functional tRNA pool, representing a safeguard to avoid detrimental effects of tRNAs with erroneous integrity on protein synthesis. Furthermore, these data indicate that the RNase T-mediated end turnover of the CCA sequence represents a means to deliver a tRNA to a repeated quality control performed by the CCA-adding enzyme. Hence, originally described as a futile side reaction, the tRNA end turnover seems to fulfill an important function in the maintenance of the tRNA pool in the cell.