A small plastid-encoded RNA (spRNA, 218 nt) has been detected in tobacco. The corresponding locus (sprA) does not contain any open reading frame and is actively transcribed from its own promoter, as shown by ribonuclease protection assays using in vitro capped RNAs. Gel-shift and UV-crosslinking experiments showed the formation of a specific complex between spRNA and chloroplast polypeptides. The mobility of the complex was further shifted when a transcript bearing part of the 16S rRNA leader sequence was added to the incubation mixture. Glycerol gradient fractionation of a chloroplast lysate indicated a preferential sedimentation of spRNA at 15-20S and 70S. These observations, and the potential base-pairing with the leader sequence of pre-16S rRNA, suggest a role for spRNA in chloroplast ribosome biogenesis, i.e. 16S rRNA maturation. By sequencing of tomato plastid DNA and heterologous northern hybridizations, the presence of sprA homologs and their expression in a number of dicot plants have also been shown.