Control of the synthesis of ribulose-1,5-bisphosphate carboxylase/oxygenase large-subunit in cotyledons during dark growth of sugar beet seedlings

The expression of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) large subunit in cotyledons was investigated during germination and dark growth of sugar beet (Beta vulgaris L.) seedlings. The levels of large-subunit (LS) Rubisco were determined by immunodetection on western blots of SDS-PAGE-separated proteins, using highly specific monoclonal antibodies. Although rbcL transcripts of 1.6 kh were shown to be present at all stages of development, from imbibition to prolonged dark-growth, the 55 kDa LS Rubisco was not detectable in dry seeds and during early dark growth. At these early stages of development, the monoclonal antibodies detected significant levels of a 3g kDa protein. This protein was associated with the polysomal fraction and was found to disappear progressively during cotyledon development and proplastid-etioplast transition, while carotenoids, rbcL transcripts and the 55 kDa LS Rubisco accumulated. Abscisic acid and gibberellic acid, which affected the rate of germination and plastid development, correlatively modified the time-course of transition from the 38 kDa to the 55 kDa protein. Synthesis of LS Rubisco during proplastid-etioplast development is therefore shown to be regulated not only at the level of mRNA accumulation, but also at the translational level. Thus translational control is discussed in relation with ribosomal pausing, which has been described for a number of mRNAs in chloroplast systems.