The components of the plastid ribosome are not accumulated synchronously during the early development of spinach plants.

The expression of components of the 70S plastid ribosome has been determined during the first 13 days of spinach plant development. Total cellular RNA and proteins were used to determine the relative steady-state levels of mRNA for ribosomal proteins (r-proteins) by dot blot hybridization and the relative amounts of proteins by immunodetection with specific antibodies. The 16S rRNA as well as mRNAs for 9 out of 11 proteins studied, including those for the 32 kDa polypeptide of photosystem II and the large subunit (LSU) of ribulose-1,5-bisphosphate carboxylase (Rubisco) show a marked increase at the beginning of the germination (day 5). At this time the plastid DNA content increases from 4% to 6% of total DNA content and so the plastome copy number can only in part account for the important increase in mRNA steady-state levels. Interestingly the transcripts of the rpl23 and rps19 genes show a different accumulation pattern, indicating either a differential gene transcription and/or an increased stability of the transcripts. In the western blot analysis a group of r-proteins can be detected in dry seeds or after 24 hours of imbibition while a second group of proteins accumulates after 3 to 5 days of development. The differential accumulation pattern of r-proteins and mRNA for r-proteins indicates that post-transcriptional control plays an important role in plastid r-protein synthesis.