In Escherichia coli the poly(A) tails of messenger and rRNAs are a major determinant of RNA stability. These tails are formed primarily by poly(A) polymerase I (PAP I) in wild-type strains or by polynucleotide phosphorylase (PNPase) in PAP I-deficient strains. In Streptomyces coelicolor it has been shown that mycelial RNAs display biochemical characteristics consistent with the presence of poly(A) tails. To confirm the occurrence of polyadenylation, rRNA and mRNA transcripts from S. coelicolor were isolated by oligo(dT)-dependent RT-PCR followed by cDNA cloning. One of the clones obtained was polyadenylated at a site corresponding to the mature 3' terminus of 16S rRNA, while two 23S rRNA cDNA clones were polyadenylated at precursor processing sites. Other clones identified polyadenylation sites internal to the coding regions of both 16S and 23S rRNAs, and redD and actII-orf4 mRNAs. While most rRNA cDNA clones displayed adenosine homopolymer tails, the poly(A) tails of three rRNAs and all the redD and actII-orf4 clones consisted of a variety of heteropolymers. These results suggest that the enzyme primarily responsible for polyadenylation in S. coelicolor is PNPase rather than a PAP I homologue.