Polyadenylation at the 3 terminus has long been considered a specific feature of mRNA and a few other unstable RNA species. Here we show that stable RNAs in Escherichia coli can be polyadenylated as well. RNA molecules with poly(A) tails are the major products that accumulate for essentially all stable RNA precursors when RNA maturation is slowed because of the absence of processing exoribonucleases; poly(A) tails vary from one to seven residues in length. The polyadenylation process depends on the presence of poly(A) polymerase I. A stochastic competition between the exoribonucleases and poly(A) polymerase is proposed to explain the accumulation of polyadenylated RNAs. These data indicate that polyadenylation is not unique to mRNA, and its widespread occurrence suggests that it serves a more general function in RNA metabolism.Cellular RNA molecules have long been divided into two groups. The first group, the stable RNAs, includes mainly rRNA, tRNA, and a variety of other small RNAs, and represents Ͼ95% of the total cellular RNA population. These RNAs have long lifetimes relative to the generation times of the cells in which they reside. The second group, unstable RNAs, consists primarily of mRNAs, which are a small fraction of the total RNA population and have half-lives that are usually much shorter than the generation time of the cell. The unstable RNAs generally contain a poly(A) tract at their 3Ј ends that contributes to their turnover; for mRNAs, poly(A) contributes to their role in translation (1-5). However, polyadenylation of stable RNAs has rarely been seen, and has not been considered important for stable RNA metabolism.In previous studies of the maturation of tRNA (6, 7), 5S RNA (8), and other small, stable RNAs (4.5S, 6S, M1, and tmRNA) (9) in Escherichia coli, it was shown that exoribonucleolytic trimming was a necessary step in the formation of the 3Ј termini of these various molecules. Thus, in the absence of the requisite 3Ј to 5Ј exoribonucleases, precursor products with extra nucleotides at their 3Ј ends accumulated. In some instances, however, these extra sequences were longer than expected. For example, 5S RNA is known to be released from long rRNA transcripts by RNase E endonucleolytic cleavages that leave three additional residues at each end of the processing intermediate (10-13). However, products were found to accumulate in some exoribonuclease-deficient strains that contained as many as 10 extra nucleotides at their 3Ј ends (8). Likewise, in a recent study of the maturation of M1 RNA, the catalytic subunit of RNase P, products with up to six additional 3Ј residues were observed (9), despite the fact that RNase E is thought to cleave this precursor at a position only one or two nucleotides downstream from the mature 3Ј terminus (14,15).In this paper we provide an explanation for these unexpected observations. To do so, we determined the nucleotide sequences at the 3Ј ends of the 5S and M1 RNA products that accumulate in the multiple exoribonuclease-deficient cells. This was acco...