Plastid 70S ribosomes were prepared from heterotrophic cultured cells of tobacco (Nicotiana tabacum, BY2), and the 5' termini of the 16S rRNA molecules present in the ribosomes were analyzed. RNase protection and primer extension experiments showed that a minor fraction of the 16S rRNA species carries a leader sequence of 30 nucleotides, coinciding with a putative RNase III cleavage site. The results suggest that an RNase Ill-like activity is present in plastids and that ultimate 5' maturation of 16S rRNA takes place within the ribosome.
