THE EXISTENCE OF PRE-MATURE 16S RIBOSOMAL-RNA SPECIES IN PLASTID RIBOSOMES

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