Molecular basis for decline of leaf function during senescence: Analysis of major determinants causing decline of the amount of Rubisco

1)Rubisco的合成在扩张过程中活跃，但在完全扩张之前，Rubisco的合成量迅速下降。在秋季扩展时，它已经下降到最大量的五分之一左右，并在衰老期间进一步下降到约十分之一。rbcL和rbcS mRNA水平的变化几乎与Rubisco合成量的变化相协调。因此，Rubisco合成的量主要由rbcL和rbcS mRNA的水平在整个生命周期的叶。衰老过程中Rubisco的降解远比Rubisco的合成活跃。由于衰老过程中Rubisco的合成对Rubisco的含量几乎没有贡献，Rubisco的降解是决定衰老叶片Rubisco含量的主要因素。2）叶绿体或完整叶绿体裂解液中产生的活性氧直接将Rubisco大亚基（LSU）在Gly-329处断裂成37-kDa和16-kDa的片段。通过延长孵育时间或增加Fe^2+浓度，LSU也可以被切割成其他几个片段。所有的切割位点都在或非常接近活性位点，就在金属结合位点周围12 μ m的半径内，这表明邻近性和有利的取向可能是决定切割位点的最重要的参数。GS 2也被活性氧以与LSU相同的方式降解。

1) Rubisco was actively synthesized during expansion, but the amount of Rubisco synthesized rapidly declined just before full expansion. It already declined to about one fifth of the maximal amount at the time of fall expansion, and further declined to about one-tenth during senescence. The changes in the levels of rbcL and rbcS mRNAs were almost coordinated with those changes in the amount of Rubisco synthesized. Thus, the amount of Rubisco synthesized was primarily determined by the levels of rbcL and rbcS mRNAs throughout the life span of the leaves. Degradation of Rubisco became far more active than synthesis of Rubisco during senescence. Since the synthesis of Rubisco during senescence scarcely contributed to its amount, the degradation of Rubisco is the major determinant for the amount of Rubisco in the senescent leaves.2) The large subunit of Rubisco (LSU) was directly fragmented at Gly-329 into the 37-kDa and 16-kDa fragments by reactive oxygen species generated in the illuminated lysates of chloroplasts or intact chloroplasts. The LSU could also be cleaved into several other fragments by increasing the incubation time or the Fe^<2+> concentrations. All the cleavage sites were at or very close to the active site, just around the metal binding site within a radius of 12Å, indicating that proximity and favorable orientation are probably the most important parameter determining the cleavage sites. GS2 was also degraded by reactive oxygen species with the same manner as the LSU.