Coordinate, Organ-Specific and Developmental Regulation of Ribulose 1,5-Bisphosphate Carboxylase Gene Expression in Amaranthus hypochondriacus

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Abstract
The expression of the genes encoding the large subunit (LSU) and small subunit (SSU) of ribulose 1,5-bisphosphate carboxylase (RuBPCase) was examined in roots, stems, cotyledons, and leaves of amaranth during the development of these tissues. The highest accumulation of LSU and SSU polypeptides occurred in cotyledons and leaves. Their steady state levels were approximately 20-fold lower in stems, while in roots neither LSU and SSU polypeptides nor their respective mRNAs could be detected. In cotyledons and leaves accumulation of these two polypeptides reached peak levels during the expansion stage of each tissue and then declined, reflecting changes in the synthesis, not turnover, of these proteins. In cotyledons and stems, the rates of synthesis of LSU and SSU polypeptides correlated with the levels of their respective mRNA, suggesting regulation primarily at the transcriptional level. In contrast, the dramatic and specific decrease in the synthesis of these two proteins during the last stages of development of the leaves could only partially be accounted for by the modest reduction in their mRNAs. Neither the translatability of these mRNAs, as assayed in cell-free systems, nor the stability of LSU and SSU polypeptides were altered, thus implying that control was being exerted at the translational level. During the development of these different organs, the expression of the LSU and SSU genes were generally coordinately regulated both at the levels of protein synthesis and mRNA accumulation.