Ribonucleic acid synthesis in chloroplasts

Abstract

Chloroplasts isolated from young spinach leaves incorporate [³H]uridine into RNA. This incorporation shows an absolute requirement for light and does not occur in lysed chloroplasts. Fractionation by polyacrylamide-gel electrophoresis of the RNA synthesized in vitro reveals a major discrete product of molecular weight 2.7×10⁶ and two minor products of molecular weight 1.2×10⁶ and 0.47×10⁶. These discrete products are super-imposed on a background of polydisperse RNA. The incorporation of ³²P_i into chloroplast rRNA species (mol.wt. 1.05×10⁶ and 0.56×10⁶) in excised spinach leaves proceeds after a distinct lag period compared with the incorporation into cytoplasmic rRNA species (mol.wt. 1.34×10⁶ and 0.7×10⁶). Incorporation of ³²P_i into chloroplast RNA species of molecular weight 2.7×10⁶, 1.2×10⁶, 0.65×10⁶ and 0.47×10⁶ proceeds without such a time-lag. The kinetics of labelling of the individual RNA components is consistent with the rapidly labelled RNA species of molecular weight 1.2×10⁶ and 0.65×10⁶ being precursors to the more slowly labelled rRNA species of molecular weight 1.05×10⁶ and 0.56×10⁶ respectively.