Stationary phase-specific mRNAs in Escherichia coli are polyadenylated

Polyadenylation of Escherichia coli specific mRNAs has so far been studied primarily during the exponential phase of growth. As part of an investigation of the polyadenylation of E. coli mRNAs in different physiological contexts, we studied mRNA polyadenylation in stationary phase by preparing a cDNA library from stationary phase RNA using oligodeoxythymidylate primers and analyzing the nucleotide sequence of cDNA clones corresponding to the stationary phase-specific genes, rpoS, bo1A, and dps. The sites of polyadenylation were found to be primarily in the 3'-untranslated region, either at the putative rho-independent transcription termination site (dps) or at several different sites upstream of the putative rho-independent terminator. A few examples of polyadenylation within the coding regions were also found, suggesting that nucleolytic degradation often preceded polyadenylation. In contrast to the poly(A) tracts characteristic of exponentially growing cells, many of the uncoded poly(A) tracts associated with stationary phase mRNA were interspersed with other nucleotide residues. The observation of post-transcriptional polyadenylation of specific stationary phase mRNAs in E. coli, some of which are transcribed by the RNA polymerase associated with sigma, demonstrates that mRNA polyadenylation is not confined to the exponential phase of growth.